That type of valve often has two internal check-valves, and one may have jammed closed. They often also have an inlet filter, and that may be clogged up. Or, the thermostatic cartridge could be shot. See if you can dig out the installation instructions which will have a parts diagram. If you can't find it, you may be able to get one from the manufacturer's website, or you may have to call their tech support. Some of those valves have a lifetime warranty, so if it needs parts, they may send them to you for free...doesn't hurt to call and check.