same thing happened to me last night. This is how I did it. You'll need a pair of nail scissors, a torch and a small long nosed pair of pliers.
1. Place the fixing nut onto the scissor blades the opposite way to how they fasten on the body of the guitar (you'll see why later...)
2. Place your guitar stringside down on a table so output 'hole' faces downwards and the jack is against body of the guitar.
3. crawl underneath the guitar and with the torch (in your mouth?) locate the jack socket with the closed blades of the scissors (nut still attached). Wriggle it around til it comes out and let gravity do the work for you.
4. once it's back through the hole, widen the scissors to lock the jack in place, and push the nut from the blades of the scissors around the screw thread. turn with the pliers and lock in place.
There are youtube clips involving wire coathangers if you do a search - but that's how I did it...
JimBob, January 2014