I have what looks very much like model 82683 PosiTemp (http://www.moen.com/shared/pdf/instruction_sheets/INS898C.pdf) The difference is on the website the handle appears round but mine is tear drop shape. This is a 2 year old house and this particular bath tub was not used for many months. Now the hot water in the shower has stopped but cold water flows fine. The sinks are also fine for both hot and cold.

I believe the suggestions largely were replace the cartridge or clean things up inside. Some clarifications:

I think I have to turn off the water supply from the heater into the house to do that? What about cold water? I am also having difficulty prying open the first plastic cover on the tear drop shaped handle. Is there a particular spot where I should put the screwdriver?

Thanks!

Use a pocket knife to pry the cap. There will be a high limit adjustment under the handle to adjust the rotation of the handle for hotter water. Adjust it high enough that it is extremely uncomfortable to hold your hand under but still can.

It sounds very much like the balance spool in the cartridge has become locked up due to lack of use. There is a kit available (part number 96988) that has a tool that is used to help remove the old cartridge. At this point, I have to stress that this kit will only work with the posi-temp valve. This handle will only rotate for on/off and hot/cold. If you handle push/pulls for on/off, then this is not a Posi-Temp.
Going forward, if this is a posi-temp, use the twist tool that comes in the kit, and take a look at the instructions that come with it. When you remove the cartridge, you can try tapping the cartridge against a hard surface, to free up the spool inside. When the spool is free, and you shake the cartridge, you will hear a rattle, similar to the sound you get when you shake a spray can. When you hear this, you can install the cartridge back in the valve. The valve itself requires fairly constant use so the spool doesn't lock up again. Just go in a couple of times a week, and turn it on over to the hot side, run it for about 10 secs, then turn it off again.

Yes, you need to turn off the cold water too. Both hot and cold supply lines run to the faucet.

New deva shower fitted worked fine for six weeks then the hot water stopped flowing.Header tank feed
Cistern for hot water.
Any ideas.
Thanks