To diagnose a toilet problem, you will need to examine the toilet to see what the problem is. If the toilet is not flushing, then you will need to check to see if the water supply is turned on and that the toilet is not clogged. If the toilet is clogged, then you will need to use a plunger to try and unclog it. If the toilet is still not flushing, then you may need to call a plumber.