Is the joint you want to reset Alyson's left hand? (That's what I'm implying from looking at your screen shot.)

If so, save her pose first. Then go to Figure > Symmetry and copy right arm to left arm; you'll be asked if you want to copy joint parameters also (you do!) This will of course overwrite her left arm pose, which is why you saved it...

Alternatively, you can save her pose and apply it to a replacement Alyson from the library. Bodies are cheap in 3D land.