Tester reads "HOT / GRD. REVERSE"
For some reason those plug-in receptacle testers show this result when there is an open neutral.

This means the neutral conductor for the circuit is open or compromised somewhere. Most often it is a poor connection at a receptacle where the wires are back-stabbed, or stuck into the "quickwire" holes in the back.

A quick test to do is to plug the tester into each receptacle on the circuit, working or not, and wiggle it. Many times the faulty connection will show as the circuit flickers or comes back on.

The best course is to at the very least move the wires from the quickwire holes to the side screw terminals. The best bet is to replace the receptacle entirely using the side screws.