Did you purchase a 30-day roaming add-on that might just have just expired? Roaming add-ons don't renew automatically. Texting sometimes lags a little behind voice and data service in this regard since all basic plans include international texting.
If that's not the case, try shutting down the phone, pulling out the SIM for a few minutes, then restarting to refresh your signal to the mothership. A further step you can try is to then manually choose AT&T as your provider in mobile networks preferences.