Pop! The shoulder strap on your backpack just divorced itself from the packbag--and it wasn't an amicable split. You have three choices: 1) Slog back to the car cradling that 40-pound load; 2) sit and wait for a Sherpa to amble past; or 3) bust out the sewing kit. Assuming you'll choose the latter, here's how to proceed.
Empty the packbag and turn it inside out (remove any stays or the framesheet if necessary). Using scissors or the tip of your knife, carefully cut away the protective seam tape to reveal the strap opening.
Poke the strap back through the opening where it was originally anchored. It should lay flat, sandwiched between the two layers of the pack's side seam.
Using your strongest needle and thread, sew three parallel lines of short, tight backstitching across the sandwiched strap. If you're carrying SeamGrip, apply a layer for added strength.
Check your warranty when you get home; most manufacturers will do a permanent repair for free.