I'm not a plumber so I am not familiar with how these things fail. I do have two of those valve in my house and I am familiar with what's inside. That valve should have a stop to keep it from going round and round. It is possible that one of the seats has ripped, cracked, or broken. Turn off the supply for that valve, remove the handle, remove any trim that covers the bonnet nut if it is covered, and then remove the bonnet nut. You may need to replace the whole cartridge or just the seats. You wont know what's up until you get a look at the cartridge and seats. The seats are cheap and available at most hardware stores. The cartridge may be a different story a plumbing supply may be your best bet. For big box stores, I think Lowes has the cartridge.