Without more details, one of the causes can be for the app limit (around 80), if you past that limit, the phone simply freezes. The solution is to acces from USB or, if available SSH, go to the Applications folder and delete one of the apps. If that's the problem, the phone unfreezes inmediatly.
You can overpass that limit installing the last Summerboard (only installing, you don't need to do anymore with it). If that's your problem, to install it without freezing, delete 2 apps instead one (you need space for one more .app before).
my iphone never starts up it freezes and vibrates like once every 30secs but itunes can find it but when i try to restore nothing happens and when i try to reset it it cant reset it coz it says 'your iphone disconnected'
please help D: i dun wanna pay the store 800$ to get it fixed D: