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How could I change it back? T-T
If you have a recent database backup and not so much activity on the forum, try a restore.
Could I change user role database using phpmyadmin?
If you're going to edit the database, make a backup!
In "gdn_user", you can set the column "Admin" to 1 instead of 0. Which means that permission checks don't apply to you anymore. You can then go into the vanilla dashboard and grant yourself "proper" admin role/privileges again.
After that, you can change that column back from 1 to 0, since you have the proper role again.