Rita,
AFAIK you cannot do that... it's easy to 'shut down', but once that's
done there's no way to 'open' it again from within itself; it's like
trying to lock a drawer and put the key in. At any rate, this would be a
half measure anyway, so I suggest you try to identify and solve the
problem, rather than circumvent it. If you post back with more details
on your database, the kind of problems you get ('stability' is too
general) and, if at all possible to identify, when this happens, I'm
sure you'll get much more useful advice than just witchcraft to deal
with the wrong problem.
HTH,
Nikos