last night i went to unlock my 16gb jailbroken 3GS and it freezes. i tried to do a hard reset by holding both the home and power button. it turned off.
now when i try to turn it back on it never goes past the apple logo screen. i tried plugging it in to itunes but it wont recognize i have it since it wont fully power on...
Just a crazy suggestion but have you tried cleaning the terminals on the docking port?
My phone got into this weird reboot loop a while back, wouldnt stay in sleep mode for more than a minute, would then wake up on it's own. If I tried to shut the phone down it would restart by itself.
I cleaned the terminals really good using this stuff called DeoxIT. Phone works like normal now. I suppose alcohol would work too but DeoxIT is electronics specific.
To restore go into DFU mode (google the instruction) and restore with itunes. (this will upgrade you to a unjailbreakable 3.1.3) if that fails, to the apple store (they might not fix it if they detect jailbreak, but they probably won't so it's worth a shot)
before you go on, wait for at least one person to like backup my post as this happened to me last time a few months ago and maybe there have been developments.