Try bypassing the tailcap (directly connect the battery negative terminal to the end of the body tube). If the flashlight works, then the problem is with the tailcap. I’d check the continuity of the tailcap first by connecting a lead to the spring and a lead to the inside edge of the tailcap (where the body marries up to the tailcap). If the switch fails the continuity check, then you’ll need to disassemble the tailcap and continue troubleshooting.
If the flashlight doesn’t work after bypassing the tailcap, then the problem has something to do with the head. That would be either the driver, the led, or the connections.
In any case, if you get to this point, I suggest you seek help here: BLF A6 FET+7135 Light Troubleshooting and Mod thread