Jailbroken iPhone 3g problems

Got a jailbroken iPhone 3g version 4.2.1 baseband 06.15.00
I tried to access cydia on it and a black screen just loads and then it closes. I tried to find the a solution but haven't had any luck. Any help on how to fix cydia or on how to unjailbreak then rejailbreak my iphone would be greatly appreciated.
Cheers

Anybody got any suggestions?

Anyone? Please desperate to get this fixed as trying to unlock the iphone so I can actually use it

Is it by holding shift and then selecting the fw I do that yeah?