A few things I would check to get to the root cause....
First thing I would do is take the water supply line to the pump off (the one ruining from your tank to the pump) and see if I could suck some water out of it... If so you can eliminate that part of the system...
If that checks out next step is pull the water line coming out of the pump off start the pump and see if it's pumping water. If not and the pump runs then check out the pump itself. If it's like most you have pump diaphragms (they are little rubber plungers) that could have something fouling them or they could be shot. In either case either clear them out or note the pump model and order replacements daiphrams.
Here's a video for you which shows you some other troubleshooting routines.
If it does pump water, then your issue is downline somewhere.
Could be your by pass vales turned the wrong way when you winterised it or perhaps your low point drains are open and the water is pumping out of them.
Hope this helps.