If it was already in a different phone, and you have had it for a while since, it may just be outdated and need replacing. Unfortunately they do go after a while. There could be a change that was made to your phone (via update) or even to your provider that causes the problem.
If it still isn't working, you could try calling your carrier and see if they made any changes to their system. They "may" be able to help you get the sim to work with their new changes (if it is still good). If not, then you can get a new one from them. Then you would have to place the old one in another phone and get what ever you need off of it that way.