bathtub faucet - hot water only dribbles out

the cold water is fine; also, the bathroom sink has good pressure for cold and hot. Please tell me it's something simple that I could do myself.....

It's probably something simple and you may be able to DIY but the professionals here will have to know the manufacturer and model of the faucet. A picture may help them to help you.

Where are you? Is it really cold?

assuming its a single lever, you probably need to replace the internal cartridge. if the water source is very hard it can gunk them up really bad. ive replaced 1 of mine and one for a friend and they were both less than 4 years old. seems like they are particularly vulnerable if it isnt used for a period of time, but aside from some blockage in the supply, its most likely caused by the cartridge. easy replacement - remove handle cover and lock pin and pull the cartridge out with pliers and go get a new replacement...make sure the water is off!

Like Redwood asked, sounds like your hot could be frozen to the tub only, but it depends on the layout of the bath.

Did this happen suddenly? Did the faucet feel different when you turned it on?