Your helmet is toast, you need to get a new one.
Without knowing what helmet you have, it is a 98% chance that it is rated for a single impact and therefore it has done its job.
A good rule of thumb is if you take a fall where you say "wow, I'm really glad I was wearing my helmet there" you need to replace it- even if it looks fine. The foam that keeps you safe is now compressed and it will not absorb the same impact again. Furthermore, you should always replace a helmet that is dented or cracked even if it has not sustained a fall.
And ultimately when in doubt, replace it.