What I think is happening is that your account was demoted to a standard account - something which has been seen, unfortunately, quite a few times recently.
Unless you can offer some evidence to the contrary, that does not mean the guest account is now an administrator. It means there is none.
Now for how to solve this:
Follow the instructions, for example, in this site: ...
You are going to have to boot into single user mode, so you may want to print out the instructions beforehand.
To summarize it:
1) Start in Single User Mode by holding down Command-S at startup.
2) Type the following commands at the prompt:
/sbin/mount -uw /
rm /var/db/.applesetupdone
reboot
3) Your mac will restart normally, except that it will appear like it is starting for the first time - you will be asked to create a user account. This new account will automatically be an admin. Name it, say, "admin", choose a password, and do not choose to import anything.
4) Logged into this new admin account, go to System Preferences->Users & Groups and give yourself the administrative privileges again. Click the padlock, enter the admin password, select your usual account on the left and check this box:
