Moen single-handle shower valves contain a cartridge that regulates the flow of hot and cold water. In addition, some Moen valves, including the Posi-Temp line, have a feature called scald-guard, which is an adjustable stop that limits how far you can turn the handle in the hot direction. If you are not able to get hot water from your Moen valve, the solution may be as simple as adjusting the scald-guard feature, but if that doesn't work, or there is no such feature, you'll have to remove the cartridge and clean or replace it.
Check to make sure there is no valve on the hot water line somewhere in the house that is turned off and preventing hot water from reaching the shower.
Pry off the cap of the valve handle, if there is one, with a screwdriver and then unscrew the handle from the faucet stem. Some handles, particularly lever-type, may be attached with an allen nut. If so, loosen it with an allen wrench and remove the handle. Then pry the escutcheon from the wall and remove it.
Check the posi-temp valve on the hot water inlet. If it is in the vertical, off, position, rotate it 1/4-turn to the horizontal position with a screwdriver. If the faucet has a scald-guard limiter, check it and adjust if necessary. Then replace the handle and try the faucet.
Turn off the water to the shower if there is no posi-temp or scald-guard feature, or if adjusting these made no difference. Then remove the handle and the plastic scald-guard limiter.
Use needle-nose pliers to remove the pin holding the cartridge in place by grasping it and lifting it straight out. Be sure the water is off before you do this.
Replace the handle on the faucet stem and use the handle to pull the cartridge out of the faucet assembly. Pay close attention to the orientation of the cartridge because it, or its replacement, will have to be inserted in the same orientation.
Examine the cartridge holes for signs of blockage that may be preventing hot water from passing through the valve. Shake the cartridge and listen for a rattling sound, similar to that of a paint can when you shake it. If you don't hear any, the balancing spool may be stuck.
Hold the cartridge in your hand and push the stem with your thumb. When the end cap pops off of the other end, pry the spool loose with a small screwdriver. Clean it and the inside of the cartridge, then reassemble the cartridge and shake it. If you hear a rattling sound, reinstall it in the faucet. If you cannot clean it sufficiently to produce a rattling sound, replace the cartridge.
Insert the cartridge, or its replacement, back into the valve and push on the handle while you re-insert the holding pin. You shouldn't have to force the pin. When the cartridge is properly seated, it will slide right in.
Replace the scald-guard limiter and the handle and tighten the screw to hold the handle securely to the faucet stem. Then snap the cap back on.