Why wont my Porter-Cable PC60TAG angle grinder not turn off?

There's just not that much to an angle grinder; basically it's just a motor with an on-off switch, so it seems pretty obvious that your power switch has given up the ghost.

There's an outside chance the piece you slide has somehow come disconnected from the actual switch itself; if that was the case you'd feel it slide between on and off with no resistance or any kind of clicking you would normally feel so it should be fairly easy to tell if that was the case.

Otherwise, if it was mine, I'd follow the instructions in the Porter-Cable PC60TAG Power Switch Replacement - iFixit Repair Guide and pull the switch. Examine it for any obvious physical damage and if you have a voltmeter, verify continuity when it's in the on position and an open circuit in the off position.

Assuming you find the switch at fault, you can go ahead and locate a replacement. If it tests out good, trace the power wires connected to the switch and look for any place they are shorted together.

Best of luck and let us know what you find!