What happens on an expired visa while still in ...

Hi,
In that case your stay is ilegal (as if you crossed the border without permission, well, not exactly).  If you want to avoid emberesment better extend your visa.  Authorities tend to be very strict and formal.  Better avoid the troubles.
Best regards,

What happens if my visa expires and I'm still staying in US.

I know this is illegal, but I could not file my extension in time and hence the problem.

Yes you would have had to start extending your visa one month before it expired, cost is 75 dollars, for 3 months, the choices are return to your own country and return with a full 6 month visa,