2001 Vauxhall Astra Driveshaft has popped out

Hey :) I was driving up a small hill around the corner from house luckily and I just lost power from the engine and the clutch felt as if it was in neutral even though I was in gear. I popped my head underneath to see if it was the gear linkage, and I spotted that the drive shaft on the drivers side has popped out of the gearbox! By half inch to an inch. I took the wheel off and tried smaking the center nut with a mallet hoping I could pop it back in, but it didn't move one bit! Please can somebody tell me how to get it back in please. :) I would appreciate it very much :)
Thanks, Craig