Tried to upgrate, now website is down

try deleting .maintenance file from public_html.

Where can I find that in Filezilla?

Have you tried: – deactivating all plugins to see if this resolves the problem? If this works, re-activate the plugins one by one until you find the problematic plugin(s). –
Using Ftp rename plugins folder to say plugins-hold and create a blank plugins folder then try logging in

If you can log in you will see all plugins deactivated go back in ftp delete the blank plugins folder and rename plugins-hold back to plugins.

Activate and update one plugin at time to see which one is causing the problem

switching to the Twenty Ten theme to rule out any theme-specific problems? –

re-uploading the wp-admin and wp-includes folders from a fresh download of WordPress?

Filezilla is ftp program you search Filezilla and install on your computer is free.

inside public_html

No You need to install filezilla on your computer is program we use to transfer files and folders to server

Thanks everyone, problem solved! I had to download 3.01 manually in Filezilla, after that it worked again. PHEW!