the galaxy S II has had many problems from my experience with this and other similar software issues. it not your fault its just a software defect in the phone. i would call the company and tell them to replace your phone. the S3 and S4 so far have not shown those kinds of issues in my experience nor has the S1. but in the case that you are stuck with the phone.
1) check battery with a store and see if its still in good condition.
2) try to get it into bootloader and do a restore/factory reset. (you will loose all your data unless you backed up on your google account or SD Card.)