Made a xubuntu boot usb drive and installed without any problem a new and fresh Xubuntu 18.04.1
on new partitions. Remove xubuntu 16 and install xubuntu 18.
No errors about disk dammage or whatsoever.
So I think now the drive itself is OK.
Rebooting the machine: immediatly into BIOS and no possibility to change boot priority, only bios-setup 
and no way out.
Before I go to your quick fix ( i can do it but i don’t know what I’m doing).
Is it possible that the BIOS is dammaged?