Hello
First you'll need to reset your password, if it's asking for one, then it's not accessing your voicemail at all. If you have a myAT&T app on your phone, you can do it from there, sign into the app, select "Profile and Account Settings" and then "Reset Voicemail Password". You can also reset it by dialing 611 from your phone, then select the option Five for "Voicemail Help Menu" and One to reset the passcode. If your messages are still in the system, they will appear.
Thank you,
Dmitriy