How to regain control over a Samsung Galaxy S II with a Boot loop?
Enter recovery mode and do a factory reset and wipe cache and install any ROM you like.

It worked for my S2 which got stuck during boot-up. I kept flashing my ROM without factory reset and wipe cache and nothing happened. 

It went back to normal when I did those procedure above. Hope it will solve your problem. My S2 is fine and running again. I almost gave up on it.