Thank you for your reply!
I tried this already, but it did not wirk out.
I think I explained myself a littel wrong:
All the files (.SchDoc(s), .PrjPCB, .PcbDoc, .PcbLib) were always saved in the same folder within Altium.
I moved all those files with Windows Explorer into an other location and opened the .PrjPCB from there.
The .SchDoc that I am referring to is not deleted. Its size is 395kB but when I open it (with the .PrjPCB or alone), it shows up as an empty schematic doc in Altium. Even when I update the PCB I do not get a room for this schematic sheet and no components from it.