Phone freezes when I turn it on?

I've had my LG GC900 for 4 months and I've looked after it well, ie, it's never been dropped on the floor or in water or anything.
Except a couple of days ago I lost signal for hours on end so I turned it off and tried turning it back on, and it just froze on the little LG symbol you get when it first turns on. I kept trying to restart it and it just did the same thing.
It's not a problem with the phone because it works fine with other sim cars, and I also tried the sim card in another phone and it froze.
The contacts on the sim look clean and they haven't been worn away. Can you help?