G4 Laptop Mac Osx Data Loss & Reset to Default After Turning Machine Off


hey there,

I just had this terrible thing happening:
all my data, that were safed onto the desktop have disappeared, after I turned my computer off and then on again.

I had to skip the filevault, forced the machine to shut down and turned it on again to get back to some files. But after I logged in again the desktop was set to default (applications were missing in the dock), my datas were gone, - including music......
I -tunes needed a new license agreement...etc..... as if I just got the computer.

A- can anyone tell me what has happened here?
B- is there any chance for me to get the datas back?

Thank you for any serious answer!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!