In order to better help you, we need more info about your issue:
What type of phone are you using?
What version of the OS? Did it update recently?
When did you activate your sim? Did you port a number?
What plan are you on?
What works and what doesn't (calls in/out, texts in/out, data).
My phone is not working I had a SIM card it work before but today it's not working?
If you had a PM sim and a plan and everything worked fine...perhaps phone is broken. Take the sim out and try it in a known unlocked phone. If it works, do some troubleshooting on your phone.
Let us know.
- Try taking it out and inserting it back in
More information about the Model of the phone etc. would be helpful to resolve this issue