If your Samsung i9100 Galaxy S II is not good for work, please try the following ways to repair your device:
You need to charge your battery, remember to backup your important data and in most cases take out SIM and SD card. It is always very important to backup online, because it is impossible to recover your data after doing a hard reset. For all of your data, a backup should be done on an external device, hard drive, raid system or media. Ensure that your important data is backed up in a safe place.
Also you can follow next to reset Samsung i9100 Galaxy S II:
press the Menu key
– choose Settings
-scroll to and choose Privacy
-scroll to and choose Factory Data Reset
-choose Reset Phone
-choose Erase Everything
Or try entering the code: *2767*3855#
And always remember that this will reset everything back to factory default settings, remember to backup your Samsung i9100 Galaxy S II important data at first.