I stopped service for a short time and then started back with internet service with a new account. When I log in my account is still under my old account number and I have no access. How do I change my account number
When you open a new account they assign a new account number. I don't think you can change the number.
Double check your UserIDs on your home system with the Lookup tool at https://idm.xfinity.com/myaccount/lookup. After you enter the requested information you'll see a list of the UserIDs on your account and an indication of which one is Primary. Be aware that UserIDs online don't always correspond with name(s) on your billing statement.
If your ID is not listed or you still can't access the new account number, contact one of their reps at the phone number on your bill or 1-800-Comcast, or use one of the other options on https://www.xfinity.com/support/contact-us/ and have them fix the problem.