Try another known 3G working, network compatible SIM card in your phone and see if it works.
Try your SIM card in a known 3G working, network compatible phone and see if it works there.
Basically swap the SIM cards between two same network phones and see if the problem stays with your phone or moves to the other phone.
If the problem (no 3G signal) moves to the other phone you have a faulty SIM card. Contact your service provider about a SIM card replacement.
If the problem remains with your phone then there is a problem with your phone.
If the problem is in your phone, backup the phone, using the phone's backup feature and perform a factory reset, using the phone's factory reset (or factory restore) feature. Once the reset is complete see if the phone now works with 3G.
If it doesn't then there is a hardware fault with the phone. How long have you had the phone? There is a 12 month manufacturer's warranty on the phone. If you have had the phone from "new" for less than 12 months, verify the purchase date and then consult the warranty statement in the documentation that came with the phone as to what you have to do to make a claim for a warranty repair or replacement. If you cannot find the documentation here is a link to the warranty provisions for the USA http://bluproducts.com/images/US_Limited...
If it does restore the phone using the backup you made earlier and then make sure that the 3G still works.