I have 3 showers in my house, 2 of the showers function fine but 1 of them doesn't. The problem is that this shower never gets hot, not even warm. It's a moen brand faucet and I could really use some help with this, thanks.

Is the hot water pipe hooked up?

I believe so, when I first moved in to this house (about a year ago) the shower used to have warm water but then the plastic faucet handle broke and it's never been the same.

First remember that a Moen and all its parts are guaranteed for life so a replacement cartridge will be free if you need one. You hot inlet port is likely clogged with some debris. Hot water debris frequently comes from some issue with the water heater so remember to flus, not just drain, your water heater once or twice a year. Let us know if you need directions on that because you should do this before fixing the valve. Your Moen may have integral stops for both hot and cold or you may have to shut down your main. Either way shut of the water and remove the cartridge. Clear the port with a tooth pick or small soft brush. Cover the open mouth of the valve body with a towel and have someone turn on the supply to flush more debris. Gease the cartrdige with silicone grease and reinstall. You should be good to go. When you install the cartrdige you will see a H/C on the brass, make sure it is right side up and on top.