How can I restart my Apple Pay?

To restart it, or see why it is restricted, you have to follow the directions in the link above. Here is the pertinent information. Basically, you have to contact Apple directly. You are posting to a user to user tech forum. We cannot resolve this for you because we do not have access to your financial records.

If your Apple Cash account is restricted or locked

To prevent fraud, ensure account security, and comply with regulations, your Apple Cash account may be restricted or locked. Your account may also be restricted if you dispute an unauthorized transaction with your bank and a chargeback is filed. Your account restriction or lock may be temporary until you verify your identity or while your account is under review, or you may need to contact Apple Support for next steps.