And you are having some of the cold weather that the rest of the country is having, you may have a frozen pipe going up to the shower head.
If so, remove the cover plate for the Moen faucet and use a hair drier to get some heat into the area. But be observant, if the pipe thaws out, it may have already cracked and have a leak, in which case, you will need a plumber to frozen/burst pipe.
Good Luck!
Dodgeman