Why wont my imessage let me sign in

i just got this ipod about a few weeks ago and at first it worked fine, i set up my icloud and signed into imessage but one day it wouldnt let me send any texts to another iphone user, and then the next day it just signed me out and didnt let me sign back in. i tried signing back in but it would just load and then take me back to sign in again. i dont know what to do so it would be nice if someone could help me.