If i stop taking Xanax what will happen?

Depending on the daily dosage and the length of time one has been taking them, the withdrawal symptoms can be quite severe and very unpleasant. It's recommended tapering off gradually to minimize any withdrawal symptoms.

I have to add to this answer that is not the "unpleasant symptoms" you have to worry about. it is extremely dangerous to just stop taking any Benzodiazapine. Yes you will feel horrible but when you just STOP taking them your blood pressure goes up. You will most likely have a seizure at best or have a fatal stroke. It is nothing to fool around with. If you need to stop taking them it should be in the hospital in a detox center, that is the only safe way to stop taking any Benzo.