Administrator permissions – need help asap

Hi all, been researching this for ages and just cant seem to get it to work.. i have no idea where i went wrong..

when i try and login to the site with an admin account i get the following error :

“You do not have sufficient permissions to access this page.”

ive tried disabling the plugins folder
reverting back to the twentyten theme
changing roles of users in myPHPadmin
changing the database version

and have had no luck with anything… i really need to get this sorted asap its stressing me out so much!.. ive looked through countless posts and tried all i can but have had no luck…

any help will be much appreciated

thanks

Jack