my imessage issues

My imessage has stopped working on and off. I tried to turn it off and back on in settings, but now it says "waiting to activate" and won't turn back on.  What is happening?

After I turn the phone off, the apple ID screen will come up and ask for my password to sign in. However, it then comes up saying that "network is unavailable".

I've tried it on 3G (with 4 bars of service) and on wireless, neither work.