Hi, i upgraded my iphone 3g to ios4.0 and didnt like it much so i downgraded to 3.1.3 when i tried turning on the wifi it sai NO WIFI greyed out! i'm going crazy and i cant fix it! i've tried all the reset options but no luck! can anyone please tell me how to fix this problem?

Thanks!

PS- if there is a solution can someone tell me a step by step fix?