Windows wasn't booting because of boot device, lost(?) HDD afterwards

Apr 9, 2018
6
0
10
Hey all.
Few nights I turned off the computer like I do every night.
When I turned it on later, the error "Reboot and select proper boot device or insert boot media in selected boot device and press a key." showed up. Restarted the computer a couple times and nothing.
I loaded the BIOS and both the HDD and SSD where there. Tried to reconnect lots of cables, nothing. Loaded Windows 8 disk, tried to format and every other option and nothing worked, except for the auto repair something. When the PC booted, the SSD, where Windows is installed was fine (I guess), but the 1TB HDD was gone. Today, when I turned it on again, the HDD was back, with 964GB of capacity, but wasn't being detected by HDD Guardian or Samsung Magician.
After restarting the computer, the HDD was gone again, Windows also doesn't recognize in the control panel.
What happened to my computer??