Hack? Please help me, don't know what to do

Hi,

two days ago all my plugins disappered from my backend, but not on the server. I tried to reactivate the plugins, but wordpress don’t show them at all. There is the same problem report for all plugins: “there is no valid header”.

I found a mysterious new user, that I didn’t add on my own. This user has administration rights. What is really weird is that this unknown user has no user name and no email address. Could this be a hacker attack?

I’m really afraid, could someboby please help me asap?
Thanks a lot, Denise.

PS: I immediately changed the rights of the new user from admin to subscriber.