I cannot sign in/vertify the imessage

I have an iPad 1 with imessage but i cant get it work. It worked before (the second day iOS5 launched) and i could made conversation with my friends(with phone no.). However, now i cant get to my friends iPad2 wifi(with Apple id) and those friends i could chat before. Then I turn off the imessage from Settings, and try to sign in again, but it is not success.

I type in my Apple id and pw, it comes out --> People will message you using your email address. What email address would you like to use ?

Obviously i can choose my Apple id only (xxxxxx@gmail.com). However when i press NEXT or press on my Apple id, it goes back to the previous page(it loads "Vertifyng", then go back), let me to type the pw again. And, it doesnt tell me anything goes wrong but just return to the previous page, so, i can do nothing, but type in my pw, --> NEXT, and then return to the previous page, and type in my pw again, --> NEXT, and then return to the previous page, and type in my pw...

So, what should i do 😟

thanks for your helppppp