"Pop up plug". Do you mean the kind of drain plug that's operated by a lever on the back of the faucet? Go underneath the sink and disconnect the rod that goes into the drainpipe from the lever that extends down from the faucet. Then see if you can move the rod up and down to operate the drain plug. If the drain plug works okay, you may just need to reconnect the lever and rod to get everything working satisfactorily. You might also want to pull the lever out of the faucet and clean it off before putting it back into the faucet.