WordPress asks to update the database, cannot login!

hi!
I have a problem on a site that uses wordpress.
after the last upgrade, I cannot login anymore.. the login page got reloaded over and over again when I tried to login. (no password error)

I downgraded the wp version via ftp, but now it says
“an update of the database is required” and when I do it, it reload the same page!!
I cannot reach the admin panel..
any ideas?

thanx so much!