It may be corrupted data. In that case, first try removing the battery for a couple of minutes. If that doesn't do it, the next step would be to clear cache or do a factory reset. Shut the phone down, then press and hold "volume up" and "home" keys, and while keeping them pressed press and release the power key. That should boot you into recovery mode, from where you can clear cache, and if that doesn't work you could try a factory reset (which will erase all data on the phone, so you will lose anything that isn't backed up).
That's if it's data corruption. If the system software is corrupt (e.g. an update failed) you can reflash that. However, I don't know Samsungs, so can't really advise on that, beyond the fact that people use software called kies, or another utility called odin, to do this type of thing with Samsungs.
This thread at xda-developers has links to software for many different carrier-branded phones, which can be flashed using odin, but that's all I know about this.
Those are the things you can do yourself. I can't rule out a hardware problem - I don't know the S2, but I've known other devices develop hardware problems, e.g. overheating, which end up with "boot loops" like this, so have to assume that a hardware problem is a possibility here as well. So if neither a factory reset nor a software reflash will fix it then you'll probably have to find a repair centre.
Good luck!
