The pump running all the time means that it's not priming.
Sine you say it's worked previously and you've not changed anything, I suspect either a fitting on the suction side of the pump has come loose, or the suction strainer on the pump is clogged up. It could possibly be a defective pump, but check the most likely stuff first.
Check the fittings between the pump suction and the fresh water tank. Then remove the suction strainer and check it for debris.
The suction strainer will either be like the one below, or it'll be a small screen in the end of the fitting on the pump.