Easiest to harder..
1. What Cortexburn said...take the base plate off the grip...ensure the terminals are plugged in. You can also take the motor out...reconnect the terminals and pull the trigger. If it spins...it's getting power and you've got a problem somewhere else (although I bet it doesn't because otherwise you would have gotten a really hot motor or some kind of thunk with a trigger pull).
*NOTE...once past (or close) to semi....it doesn't matter what position the selector is in. It'll shoot semi up to a certain point and then go into full. Being halfway inbetween doesn't do anything harmful.
2. If the motor doesn't get any power...then the issues is somewhere along the electrical path.
- check the battery/wiring connections...press the wires back into the connectors...when connected, press the wires together. If you get power then your connectors have loosened up...repair/replace.
- if it's not those two (and if you really don't have a fuse...one guy said he didn't but really did, he just didn't know was was in the little black plastic tube)....then you're into removing the mechbox and checking out the switch/wiring.
- little metal post that sits behind the switch block (little block that's attached with a spring and is pushed into the electrical fins)....if it's broken, the block will sit back too far and your trigger won't pick it up. Full trigger movement....but no action/power. If that's the case...get a new mechbox shell or mechbox or repair it with a pin. It's not your buddy's fault...it's just dumb luck (but you can legit say he owes you a beer).
- burned out metal fins in the switch...if they're black, they're burnt. Replace the switch...consider a MOSFET...etc...
- a gear has chewed the wiring...replace wiring harness
- a broken gear tooth has jammed everything (probably not the case since the motor would spin in the above tests)
- the piston is jammed (probably not the case)
Best of luck