No power for a long period of time can cause the airbag light to come on but IME it can take weeks or months with no power for that to happen.
Reseting the the system can be done by using a Scan tool with that ability or the manual method.
Here's a couple of previous discussions:
Here's the FSM diagnostic proceedure courtesy of Raventai. 2. Using diagnosis check wire: CHECK DTC (a) Output the DTC. (1) Turn the ignition switch to ACC or ON position and wait approx. 20 seconds. (2) Using SST, connect terminals Tc and E1 of the DLC1. SST 09843–18020 NOTICE...
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my airbag light is has been on for the roughly the past month, I don't know what caused it. Just appeared to be on one day while i was driving. I already did the reset where you ground the connectors in the DTC. After I do the procedure, the light goes off and then comes back on maybe 5 seconds...
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Airbag SRS light reset resetting procedure