We are on city water, but also have a well for 2 of the outside hose outlets. I used the well to fill our 1700 gal above ground pool, which was fine until it was about full. The water pressure was erratic and then no more water, so I turned of the pump/compressor. Does this mean that I just depleted the reserve water in the cement cylinder and the pump couldn't keep up? I left it off for about a week, and it seems to be pumping water again.

I would guess you depleted the well, especially since it seems to have recovered.