As Kev said, if the cork is not damaged, it's just flat, then try a little steam heat. You won't need much so be careful.
Like Stefank said though, it's not hard to replace the cork. You can either buy a preformed "cone" of cork that you just glue into place (Don't ask me what sort of glue to use) or you can buy a flat strip of cork and do a true DIY job. If you have a tech nearby, I don't think they charge much either. Failing that, PM Griff136, he' our resident tech and see what he suggests.
If your cork is damaged and you want to carry on playing then you could just wrap a small piece of paper around the cork. It may just fill the gaps!
Best of luck.