iMessage not working, text only??

When I send an iMessage (blue) on my 4S it briefly says "delivered" then quickly changes to not delivered and I have to resend it as a regular text (green) Then that goes through. The imessage on the iPad is also not going through and doesn't allow me the option of sending as a "regualr" text.. any idea what is going on?? I'm not the only one having this issue. The person I am texting with has the same isue. Any idea what is happening.. been this way since about 4:00pm today