I've had the exact same problem numerous times.
here's what you do. (least.. this is what I do when it happens)
Take out the mag and receiver, set them aside and take a look at the body of the p90 where the body meets the receiver. Theres a bent tubular metal piece (kinda like a really thick bent coat hanger) that sits inside the little plastic part which is attached to your trigger. pull the trigger slightly and you'll know what I'm talkin about, the plastic bit moves the metal piece back.
pull the metal piece back and out of its little slot gently, try not to bend anything. Then push it towards the gearbox until it fires. (might freak you out when it goes full auto, in which case, let go

then just put everything back together and it should be fine.
G'luck.