It might be the filters in the pump are clogged.

If there is a pump above ground it may have lost prime - look up priming a well pump. If you're hearing the pressure tank running, there might be an air leak in the bladder leaving it unable to hold pressure.

Never thought there were filters. Guess it makes sense, though.

Could be the impeller on the pump is shot, wouldn't normally go straight from working fine to completely busted, but possible.

Could also be no water at the intake level of the well