So my rvf400 started having a lot of white smoke coming out from the exhaust and now it's not even starting please help me

Could be many things. Give us a clue.
What was the last thing you did to the bike?
There's a good chance that human intervention had unintended consequences....

With that little of info, I'm guessing coolant in the combustion chamber...

I'm guessing choke is stuck on and motor flooded.

im guessing its fuked!! .but sounds coolant/head gasket type thang!!..shame

Well the bike is starting and running but smoke is still coming out from the exhaust. I'm thinking it could be head gasket or piston ring fuckinng up and letting oil get into the combustion chamber

What colour is the smoke?blue or thick white.check the oil and see if its a whiteish sludge which would indicate head gasket failure.as has been mentioned have you worked on it before this happened.we need a bit more infomation to give you answer!

Some more info needed although I'll echo the concerns of the others by suggesting a possibe head gasket failure.. Have a look at your oil, that will give you a clue.. Fingers crossed it's not critical..

Could of over filled the oil and its chucked a load into the airbox? Is it Making any strange noises? Prob best take spark plugs out an see what they look like

Could it be be a piston ring or an valve stem seal problem? And I think it's blue smoke