The DALL-E 2 Alpha Artist group was available for a limited period of time. Now that DALL-E 3 has come out, it seems that your access has been revoked.
If you were given access, it could have been like this (my account):
Notice how it has an expiration date. A good guess would be that your gifted Plus plan has expired. You can purchase Plus for $20 a month to continue using the tools.