If the upper dash was removed recently, it's possible that someone forgot to plug the airbag back in but then I'd think that'd be obvious.
It's unlikely that a wire loosened by itself and IMO, it's also unlikely that you'll diagnose it as the first item required to test the circuit is a test resistor (special tool) that goes in place of the air bag to check circuit resistance. Never use a VOM on any airbag or it'll blow.
Assuming that all plugs are still plugged in, your choices are airbag, wiring in between airbag and crash sensor (under e-brake handle where liquids from a cupholder can spill on it). I have yet to see where the wiring or plugs go bad by themselves and bad airbags are also rare. I have seen where early 850s with cupholders installed have had bad crash sensors causing permanent faults due to having liquids spill on them. In later years (like '95, '96) it then had a plastic cover over it to protect it.
With a test resistor, the system is checked by process of illimination going backwords to the crash sensor checking the wiring. There are no specific tests for the airbag itself or crash sensor.