I installed sucessfully Lubuntu 22.04 LTS on a Dell Optiplex 760. It restarted the firt time.
But after I turned off the computer, and reboot, it asks for a external drive, the SATA disk is not presented.
At the end of the installation process, you removed the installation media
You then hit enter as instructed
The machine rebooted like normal
You then turned off the computer
When you turned it back on, you got an error stating it could not find an OS to boot
If that is correct, could you send a picture of your BIOS settings, in particular the boot order?
In any case, you shouldn’t have to change anything. Your machine likely has one drive. In order to boot the original OS, the boot order had to be set in such a way that it would at least at some point attempt to boot that one drive. Installing a new OS won’t change that. So it should just work.
That said, assuming all of the above is true, your most likely issue is a hardware failure. Drives do die often, even after you’ve just used them.