if its the same fob others vauxhalls use, then they have a large design flaw. I've fixed many fobs which have broke, most were from omegas infact.
what happens is the battery clip comes away from the PCB. It has 3 pins around it and these usually break off. Use a knife to open it up and take a look. You can either solder these back on, or use some doubled up cardboard between the top of the battery clip and the plastic casing. this will push it against the pins.
its most likely the clip has come apart inside, but sometimes the components can fail, if your handy with electronics, you can fix them fairly easily. its just abit fiddly.
hope that helps.