You...have...to...connect...the...pump...end...to. ..the...sac. Hee, hee, just kidding.
Check to make sure the pump is turning the correct way. An aerator pump will pump water in reverse, barely.
If that checks out, lift the other end of the hose out of the water until it primes. If you start the pump with the end of the hose in the lake, it has to be able to push out the captured air bubble. And that can be difficult for older pumps.
If it still won't pump, you probably need a new one.