The pump should be easy to find and get at. If you have a pull out dining are the pump is under that. Unscrew the metal trim bar along the floor of the pull out. Could be that a hose has come off. If the pump was allowed to freeze, the plastic parts could be snapped and the pump will need replacement. Also a chance that mice got hold of the hoses.
What confuses me a little about your issue is that it occurred while filling. Was the pump turned on? Even if the pump was damaged or hoses disconnected the water should have stayed in the tank or spilled out the overflow.
Any more clues for us?
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