Hi I have been getting plenty hot water through my shower but as soon as I turn the cold on nothing please someone help \me thanks

What kind of shower faucet do you have ? Single handle ? Two handles ? What brand ?

Single handle: If it is a pressure balanced valve than the pressure balancing device inside your cartridge is stuck in one position due to presence of debris or calcification. Close main water valve, remove cartridge and clean it. If it doesn't help than purchase new cartridge

Two handle faucet: You may have chunk of debris inside or loose washer. Also, the valve stem may not be installed properly. Close water, remove stem, see if all appears clean and washer is attached. When reinstalling stem, make sure it is in open position. Install stem, twist handle to "close" position, turn water back on.

Also, some faucets have water turn off installed at the body valve. Make sure this valve is in open position.