Hi, I am trying to repair a leaking bathroom faucet. Unfortunately I haven't gotten very far. The shut off valve for the water supply to the faucet under the sink seems to be frozen/stuck in place. I cant get it to budge. I'm afraid to do too much banging for fear that I'll break something. Any tricks to getting this unstuck? Obviously we haven't turned it off or on in a ling time. It has an egg shaped knob that is supposed to turn, that's the part that is stuck.

Thank you!