We'd need to know how it stopped working in order to really help. Did it suddenly stop, did you change phones or contracts, did anything else at all happen, do other people in your area/on your network have working data? Stuff like that would help us identify the cause, but you'll have to post again to tell us.
Otherwise, a good thing to check is your Access Point Names settings (APNs). These can be found in the phone settings, under mobile (or cellular) data settings (network settings). If these are missing or wrong then your mobile data will stop working. You should be able to find the correct values for your network from the web, or else speak to your provider.