You have three options Charlie,
If you're confident and able bodied you can get up there, inspect the fixings and correct it - not always the easiest of jobs on some ceilings especially in older properties. Longer fixing screws and/or rotate the fitting a bit to get new screw positions.
A flippant alternative is to just leave it and hope it doesn't fall down - I did this last week with a hall ceiling and a four foot square section fell down - silly me.
Probably best is to summon an electrician and have it repaired safely.
Take care...
Peccavi, July 2009