If haven't since updating, you may need to sign in to iCloud (Settings>iCloud>Account, it may also ask you to agree to new terms). You could also turn iMessages off then on again (wait for it to say "iMessages can be sent...") in Settings>Messages.
Also, have you tried giving your idevice a power cycle (not sleep mode, but a full power cycle).
If you've done all of this, then it may be that Apple's servers are a bit overloaded with new activations today, as already stated.