Most likely it has gotten trapped by either the sheath rolling back on itself OR some long hair is wrapped around it.
You can put cool, NOT cold, water to help lubricate it and try to slide it back into the sheath... you may have to move it further out in order to get it back in.
If this doesn't work you need to see a vet ASAP as the swollen tissue can quickly become infected due to the shut off blood supply and this is extremely serious and can even be deadly. ASAP means tonight, NOT waiting until tomorrow as that will be too late.
Try to have someone hold him while you attempt to get it back inside the sheath as this is quite painful to the dog and he may try to snap at you or the person holding him.
Good luck!

Toto, CD, RN, CGC answered on 1/29/13. Helpful? / 0