Check the contacts between battery and gun - there could be corrosion though unlikely in something just four months old, or the spring contact may have been moved somehow. Once past those you are looking at something going inside the gun - maybe the trigger, hard to tell without knowing the unit's history.
Presumably there's an interlock switch to ensure it's pressed against something before firing, is that jammed with some debris or bent? Watch you don't accidentally shoot yourself if you're poking around the interlock and it is the problem!
Does the instruction manual not cover the meaning of the different LEDs? It's years since I've used one and it may not have been a Hilti but I don't remember it being reliable.
