When does the computer quit? Never starting at all--black screen or does it get to Dell logo? Any error message? Did you try tapping F12 immediately when you start up without waiting for any screen to open the boot menu? If you can get to the boot menu, run the diagnostics including the custom hard drive test. If you get any errors, replace the hard drive. 

Yes F12 works & there is an error code when I test the hard drive: error code 2000-0146 I don't really think there is anything wrong with the hard drive but I will try another one & see if it works. Did you see where I said somebody tried to fix it & messed with the system settings in the bios & I think he changed settings that he shouldn't have messed with. Do you know anyway to start windows 10 in command prompt? Thanks for your advice. Brenda

Your hard drive has physically failed and needs replaced. Your Windows 10 Bootable USB is likely set up to Boot using the GPT partition scheme which is not supported on the OptiPlex 780 which has a Legacy only BIOS.

2000:0146 Long test contains Previous Errors.

There are no soft fixes for Physically Bad Hard Drives.

Previous Errors

Windows 10 boot works fine on a GX620 all the way thru 780 790 980 990 etc.  There are no soft fixes for physically bad hard drives.

I tested both F12 Boot and install scenario and it works fine.

Burned 1803 DVD and RUFUS Created USB installer from the ISO used to burn the DVD.

Both work fine and install windows 10 on GX 360, 380, 620, 745, 755, 760, 780, 790, 960, 980, 990,  3010, 3020, 7010, 7020,  9010, 9020.

The issue is bad drive.