windows - Cable came loose and pc gives error now

People brought a computer here and the pc gives following boot error:

"Reboot and select proper boot device or insert boot media in selected boot device and press a key"

I checked the computer from the inside and the cable from an SSD was loose. Fixed that problem but still the computer gives me that error. I know of the MBR fixes and stuff. But I don't know what windows was running on the device. The people who brought the device doesn't know it either because it was from a person who passed away.

So how can I fix this MBR? I've read you need to know the exact windows version, but I don't know how to find this out?

kind regards,

EDIT: I found another SSD ( he has al lot of HDD's ) I plugged it in too now it gives me the error "missing operating system"