I'm assuming you're in the UK, since it's "3" and you're using English - in which case I would start by calling 07956125820 and check the IMEI (this service is operated by T-Mobile, but the database it accesses is shared across all the operators).
If it doesn't come up as being lost or stolen on that system, then it's just a block put in place by "3" and you should be able to get it removed by talking to customer service and explaining the situation.
If, however, it does come up as being lost or stolen, it is extremely difficult to get it unbarred.
Answers & Comments
It depends on how it's been blocked.
I'm assuming you're in the UK, since it's "3" and you're using English - in which case I would start by calling 07956125820 and check the IMEI (this service is operated by T-Mobile, but the database it accesses is shared across all the operators).
If it doesn't come up as being lost or stolen on that system, then it's just a block put in place by "3" and you should be able to get it removed by talking to customer service and explaining the situation.
If, however, it does come up as being lost or stolen, it is extremely difficult to get it unbarred.