SIM is vaild

Your phone is not unlocked for use with other carriers. If you want to use a different carrier, you will need to contact the current carrier to get it unlocked.

The SIM card you are trying to use is for a different carrier than the one the phone is locked to. If you want to use that card, you will need to get the phone unlocked.

Have you tested your SIM card in other devices? Does it work?

What is the exact error message that you get?

Were you using the exact same SIM and carrier before this happened?