You first will need to find out, what caused your vassals to declare war on you. The most likely reason is, that you did to many 'bad things' (grab claims, declare war, take counties, revoke titles etc.) in a to short period. Doing this will effect your reputation and your reputation effects how loyal your vassals will be, a bad reputation leads to low loyalty of your vassals and then leads to them rebelling against you.
See this thread for the effects of badboy points.
You will now have to try to improve your reputation, which usually means you have to wait till it goes down, over time.