Disconnect the discharge pipe and then plug it in. I saw one once that hadn't operated during the dry season and the check valve stuck closed. Those pumps don't develop enough pressure to un-stick a stuck valve.
It could also be air bound with air between the pump and the check valve. Many people drill a 1/8" hole in the pipe just above the pump, before the check valve, where it will discharge into the sump. That will let the air out so the pump will be have water around the impeller.
If it pumps water out the 1/8" hole but not out the discharge of the pipe, then the line is plugged somewhere.