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I have experienced your symptoms. I restored my database from a backup and that took care of the problem. In my case, it seems that the database got corrupted when I accidentally unplugged my server while I was logged into the site (yeah, yeah, I know). Hopefully that's not what happened to you.
I/we need more information:
Do you have physical access to the server?
Did the power to the server cut out unexpectedly?
Is this a new installation?
Did you recently install (or activate) a new module? Try to remember the last thing you did as administrator.
Do you have backups of your database? In the worst case scenario you can resore your database and it should work.
If you haven't been backing up your database, you might have just learned a valuable lesson the hard way. I have been using
AutoMySQLBackup for a long time and I love it. There are others. Anyway, if you haven't been backing up you should install this and back up your database. There's also a module called
Backpack to help you do this in the future.
Read the instructions for either or both of these carefully before you do anything!The worst thing I can imagine is you'll have to make a new database and a new site, then copy everything from the modules, themes, etc. folders to the new site. Then you'll have to use phpmyadmin to copy the tables you want from the old database to the new one.
When this happened to me for some reason the only damage was that I couldn't log in. All of the other data was intact.
I hope this helps.