I got this old Dell Optiplex 780 for free from an office and when the power button is held down and error code of 3 appears and the power buttons doesn't light
A reference guide to the Dell OptiPlex Diagnostic Indicators | Dell US
Dell OptiPlex 780 No Boot, Solid Green Power LED, 123 DIAG LEDs. | Dell US
Dell Optiplex 780 Late 2009 Service Manual Mini Tower Computer
Dell Optiplex 780 Will Not Boot Up - Dell Community