Nokia 2310 suddenly stopped sending text message

If the SIM works fine in other phones, there is a phone issue. See if you're eligible for a warranty replacement (eligible if purchased new from AT&T within the last year or if purchased refurbished from AT&T within the last 90 days). If ineligible and you have insurance, use it... there will be a $50 deductible applied to your next bill. If ineligible and you haven't got insurance, get a replacement through somewhere like ebay or craigslist, upgrade (never EVER use wirefly or amazon, trust me) through AT&T preferably, or purchase a gophone (AT&T prepaid) from walmart, walgreens, wherever (If you go to RadioShack, they'll do everything for you). You just put your SIM in it and it will have your phone number and contract information... you don't have to sign up for prepaid service.