1. You tripped a breaker. Sometimes it's hard to see that a breaker has tripped; the handle might stay in nearly the same position. Run your finger down them, and see if any feel wobbly or slightly tripped. Or figure out from the list which breaker it is. Then turn that breaker off, then back on again. (Note that if the pump tripped the breaker, it is likely to either have a short in it, or to be too large for this outlet. If so, it's likely to trip the breaker again when you try it again.)
2. The wires in the outlet were loose, and when you plugged in that pump, you shook the outlet and a wire came loose from the connection. So shut off the power, take the cover off, and look for a loose wire. Check them all, and tighten each one up.
3. This outside outlet was added recently, and done properly, via a GFCI connection. But it's an extension from a GFCI outlet elsewhere -- the GFCI isn't in the breaker box or at this outlet, but at some other place in between. So look in your garage or basement or wherever to find that outlet, where the GFCI will be tripped. Reset the GFCI there, and this outlet will work again.
Those 3 are roughly in the order from most likely to less likely.