Pull out the batteries, and wait a while...
While the batteries are out of the light, clean the contacts, put it together, click the tailcap, clean the contacts again.
...then put the batteries in, and try it again.
If it works again, then internal memory just got messed up, and it needed a few minutes for the
electrical charge to drain out before it could reset itself, no biggie.
IF it doesn't, I'd send fenix an email, and ask them what to try next.