I reckon that the diaphramme insede the pump has dislodged itself.
Quite simple to rectify, unscrew head, carefully remove the head itseld and check the
seating of the said diaphramme, sometimes when the pump freezes the diaphramme
(traingular rubber part usually with 3 nipples), needs taking out and placing it very hot water for
a few minutes, this will cause the rubber to take back its natural form again. Refit and everything should work again.
Had this problem a lot myself untill I put in a heater over night (next to pump and hoses) to stop this.