The instinctive reaction is sympathy. But sometimes it is misplaced. What our colleague usually needs is not just a shoulder to cry on but recognition that what is happening may have long-term effects.
At its extreme, this sudden loss of confidence may be signalling the end of a career in which case he or she will eventually need effective counselling about ill-health retirement procedures and other mechanisms for helping with the results of breakdown. But, if this is to be avoided, managers must look to effective methods of support.
Teachers are well used to hiding their feelings. "The show must go on" is as much an educational as a theatrical convention. Also, the barrier of the classroom door remains part of the culture, despite the greater openness of recent years.
But, for the distressed teacher, admitting to the pain is a necessary but early step to dealing with it. This means someone has to listen.
The problem of too much to do and too little time to do it - which we all face - means there is no space to listen to each other at anything more than a superficial level. Not only do the signs of a colleague in trouble often go unnoticed, there is a strong incentive simply to prop up the situation and so avoid the real issue.
A head of department who will take a more difficult class off somebody's hands, the year head who deals with routine issues to save group tutors the work, managers who excuse late submission of reports are all doing so with the best motives but, in fact, exacerbate the problem.
By trying to lighten the load in these ad hoc ways we unintentionally contribute to the process of denial that obliges the troubled teacher to keep acting as though nothing's wrong until a crisis.
This is the moment for which most of us are badly prepared: the teacher doesn't know why he or she can't stop crying and the head or other colleague faced with a blubbering and incoherent adult, who only yesterday appeared as together as anyone else, doesn't generally know the words or the actions that will help.
And there are usually classes waiting, parents to see, meetings to attend or budgets to be prepared. Clearing the diary sounds easy - but seldom is - and while the human instinct may be simply to listen and to hold (actually or metaphorically) it can never be real when you have one eye on the clock and the phone may ring at any moment.
So, too often, the easiest solution is to make sympathetic noises and encourage the distressed teacher to take time off. But this could have the opposite effect to what is intended. Although the classroom may be a no-go area for a while, staying home can also deepen the depression.
Being at home every day with no one to talk to (if the rest of the household is busy) requires a particular kind of strong-mindedness which, almost by definition, the troubled teacher has - at least temporarily - lost. Where the teacher's partner is also home there may be a tendency to dwell on the minutiae of school life and the breakdown's symptoms with, fairly naturally in the circumstances, the partner seeking to "blame" the school. This will not aid receovery.
For most people, mental illness still carries a stigma. Whereas a broken leg is seen as a legitimate cause of absence, the broken mind can feel like malingering. Guilt at being out of school creates additional pressure and the longer it goes on the more fearful of returning the sufferer becomes.
It's not just the fear of failure, of breaking down in front of classes or not being able to remember simple facts or procedures, but also of what colleagues are really saying. Anyone who has been away from school in these circumstances knows what it's like to doubt the expressions of sympathy or support, and wonder if there is really implied criticism.
So, anyone advising the suffering teacher has to deal with the dilemma of how best to support. If going home produces problems, won't staying at school be worse? Not necessarily. If the school environment can be shaped to help the person in trouble, recovery may be quicker.
This could mean the teacher coming to school but undertaking different or lighter duties. Attendance might be only part-time. Additional responsibilities might be temporarily assigned to someone else. Whatever the strategy, though, it should be open, understood and accepted by all colleagues.
Illness often results from a teacher feeling unvalued. A situation created to support the sick teacher indicates his or her importance in the eyes of colleagues. It also emphasises that illness is not a failure.
Whether this kind of support is possible will depend on resources but also the importance a school places on human values. All the structures of our over-pressured system - particularly those dictated by budgets and timetables - militate against it.
But, whatever its published aims, a school which cannot accommodate and help an afflicted teacher cannot truly claim to "meet the needs of individuals" or to "value each member of the community".
And what of those people to whom the distressed teacher turns - often the head or deputies? As well as time, they need skills and the inner strength to accept the burdens of another human being who is operating at the extremes of the emotional continuum. It's not a role for which their training or experience generally fits them and yet if they don't do it who will ?
This lack of guidance and the increasing incidences of teacher breakdown highlights the need for counselling of staff, at all levels and in all roles. Professional support in a context of real understanding and detailed psychological knowledge is the only answer.
But can it be afforded in a market-oriented, underfunded and undervalued service? The more important question is: can we afford not to do it? People are being damaged, and many of them lost to the profession, by a system that imposes intolerable pressures and then provides few effective means of dealing with them.
Helping people to manage their stress is important and reflected in the wealth of books and courses now available. Enabling schools to support those who have gone beyond the point of helping themselves, however, requires equal priority.
Mike Fielding is principal of The Community College, Chulmleigh, North Devon.