happen. i replaced the pump. then about two years later i decided to take the old one apart, to see if i could fix it. like you, i saw nothing wrong. i washed everything off and re-assembled it. i ran a hose from a pot of water to the input gave it power and it pumped away. i now carry it as a spare.
this summer, the new one leaked from the switch for awhile. it seemed to stop after i used the pump a bit. i guess these things like to be used.
that pump is pretty simple so i'd also suspect a piece of debris stuck most likely in the 