Galaxy S2 - Unable to perform a hard reset
Have you tried starting it in Safe Mode?

Press and hold the Power key to boot up your Samsung GT-I9100 Galaxy S II.
After the start-up animation begins to display, press and hold the Volume down key.
When the lock screen is displayed Safe mode is displayed in the lower-left corner of the screen. To boot your Samsung GT-I9100 Galaxy S II in normal mode again restart your device.

If you've installed / updated something recently, try removing it and see...

If that doesn't work, you might need to effectively re-install the OS by flashing a ROM of choice

If you can boot to recovery, flash a new ROM (or a new version of the stock Samsung ROM). Check out the relevant forum on XDA-Developers. Else, use ODIN to flash via your PC and USB cable.

Be mindful of the model and variant - the international i9100 differs from other US based variants.