have you tried pulling the battery for a few minutes?

 

Flash Ice Cream Sandwich(Any version would do), re-root it then flash http://forum.xda-developers.com/showthread.php?t=1569686 .

 

Or you can follow the instruction here:
http://androidforums.com/galaxy-s2-...gs-root/648541-jelly-bean-rom-guide-mods.html

If you can go in Download mode you have to choices: one is what SCPadgeebar is recommending, the other one is to flash directly the JB by following Calvo1 thread (i did it for one friend it works great); you can leave it stock or root it with Philz.

 

Either way you will void your warranty as XXLSJ is not an official release.