I've had this happen before. Unfortunately, my experience is that if the limbs are so badly twisted that the string pops off, it's too late to do anything about it. However, no reason not to try. With the bow unstring, twist the limb in the opposite direction of the twist and hold it in place. Repeat until the limb is straight. If it stays straight after you shoot it for a while, you're home free.
A slight limb twist is easy to correct and usually stays corrected, so long as you avoid torquing the string when your shoot the bow, which is something that most of us do from time to time. For that reason, I check for limb twist by sighting down the string from the end of the bow each time I finish shooting one of my recurves. I've never had this problem on a longbow.