There could be many reasons. We need more information in order to be able to help.
For example, is this a new phone or a new SIM, or has something recently stopped working? Does the SIM work in a different handset? When you say "network" do you mean no data connection, or no network connection at all (i.e. no voice or text as well)? What handset do you have and what network are you on?
If you could post again with as much information as possible we may be able to suggest something.