Your phone is stuck in a boot loop. Take the back cover off and spray compressed air in and around the power switch. If dust or lint is in there, it can partially jam the power switch, causing this symptom. If you have backed up your data and synced your apps with Google, it's no big deal to do a Recovery boot and clear all data, which should cause your phone to recover from its current state (unless you are rooted, in which case, it's a whole 'nother scenario and you should repost with as much info as possible.
To boot the S2 into Recovery, remove the back cover and battery, wait a minute, and then reinsert the battery. Immediately press and hold Volume Up and Power at the same time until you see the Samsung logo appear, then release. Using the on-screen instructions that appear, wipe cache, wipe data, and then reboot.
If you continue to have problems, I suggest
signing up as a member and posting to the S2 sub-forum, in order that you can have a 2-way dialogue.
