You'll have to replace the 3.5mm audio jack socket on the front panel if it's suffered serious damage. Easier said than done if your case manufacturer doesn't supply spares. You could just buy a 3.5mm socket on its own, pull off the front panel, cut out the bent socket and solder the new one in, then use epoxy adhesive to secure the "bodge". Alternatively, just buy a 3ft/1m 3.5mm extension cable and plug the headphones into the back.