Hello, in my shower hot water comes, once I turn shower faucet to cold, no water at all. I went to home depot, they asked me to change cartridge, I did so, still no cold water. So what would be the problem?

Shut the water off, open up the faucet and remove the cartridge. Now turn the cold water back on. Do you have a stream coming out of the open faucet? Let me know. Back to you, Tom

No still no cold water after taking the faucet/cartridge off

Then the problem is before the angle stop. Shut the water off at the house shut off and unscrew the compression nut on the angle stop. Remove the angle stop and place a pan over the open pipe to catch any water. Now have someone turn the water on for a few seconds while you hold the pan. Is there pressure now