Hello micromart,
The same thing happened to me, and I don't think I - or you - did it, I think it was a bug in an upgrade.
I found a fix that works. Here you go:
1. Reboot computer, at restart immediately hold Cmd + S keys down together until white text on black screen appear.
2. at prompt, type : " /sbin/mount -uw / "(w/o the quote marks) and hit return
3. at prompt, type : " rm /var/db/.AppleSetupDone " ( be sure to leave spaces as stipulated)
4. reboot
5. Set up the computer - as if it were the first time. At the "create a computer account" screen, create a temporary main user account using a new Full Name and Account Name, different from those of your original account. Also use a different password. Remember that password!
6. log in under that new account. In system preferences, change the status of your original user account from "standard" to "administrator."
7. Log out of the temporary administrator account and into your original (normal) account.
8. Delete temporary administrator account (if you want to).
I gather I am lucky in that I did not enable FileVault when I originally set up the computer. If FileVault is enabled, one can apparently recover administrator privileges but the procedure is more complicated.
Good luck!
Ann