My truck has been parked for 2 weeks and when I started it today, the air bag light flashed code 51 and stayed on...2004 with 30,000 mi.

 

Your battery may be failing or may simply have drained somewhat due to the extended parking period instead of a hard failure of the airbag (SRS) system.

Once the issue is resolved with the battery, you'll need to have the SRS system reset by either a Ford dealer or an independent with the appropriate scan tool. The Snap-On Solus or the OTC Nemisys are just two of the capable scan tools.

These tools are not cheap, don't expect a free reset.

Steve