Blog suddenly uninstalled

Hallo to everyone.

I have a problem which I can really not understand, let alone fix. I’ve had a blog up and running for a year now… last week I couldn’t update it: it just wouldn’t save anche changes I would make. I was considering updating the version and see if it would fix itself but then today a much worse problem has appeared: the blog suddenly seems to be un-installed.
Basically when I try to login the install.php page appears.
Please note that I did not touch any database or configuration page.

Can someone help?
Thanks in advance,
Slawka