how to use .sql file to restore wordpress

My attempts at upgrading my wordpress to 2.6 have completely ruined my site… I cannot access wp-admin, people can’t leave comments and also I cannot add any new content/

Basically, I backed up my wordpress database, its as a .sql file and I was just wondering how do I use that to restart my blog?

I have made such a mess of the upgrade that i need to delete all the files and basically start again – (or rather the upgrade plugin mucked it up!)

i made a theme or two so know I need to save those but can anyone recommend exactly the procedure of starting my wordpress all over again (of course with the aid of the saved .sql file)

Thanks