Try deleting your APN/internet settings and reconfiguring them from scratch.
Read and follow these instructions carefully
http://community.48months.ie/t5/The-Knowledgebase/My-Internet-is-not-working/ta-p/45928If that doesn't help then you might want to try a replacement SIM.
You may also want to contact 48 support for more advice.
See the link in my signature below.
Hope this helps.