Assuming you just removed yourself from the admin role, try to have a look at the tables admin_users and admin_role using PHPMyAdmin.
On my fresh installation I can see the entry for me (the admin provided during installation) in the table admin_users. Then in the table admin_role I have (amongst others) the following entry:
parent_id: 1 ==> this seems to relate to the entry 1 in the table admin_role, which is the administrator group
user_id: 2 ==> this seems to be the user id of the entry in the table admin_users
role_name: xxx ==> this was simply my first name - I don’t know what the purpose of this field is
So try the following:
Make a full backup of your database (or at least of the two tables in question).
Find your user_id in the table admin_users.
Then find the corresponding row in the table admin_role.
Change the value of the parent_id column to in this row to 1 (assuming this is your administrator group, which it should be).
If you are logged in Magento, log out, clear your session and log in again.
Please note that there are no guarantees that this will work - so do not forget to make a backup!!!