If you have a pressure balanced faucet, the Cartridge may have become clogged.
You can also check to see if there is a shutoff behind the escutcheon that needs to be turned.
In my guest shower I haven't used it in a while and I tried to use it the other day and when I turned the handle to hot no water came out. The pressure is great on cold, but as soon as I turn it to hot no water flow at all. I'm very new to any kind of home repair so any pointers would be helpful.