It is impossible at this time to run the Dell diagnostics until you can get past the bios post screen(the Dell Symbol). So to start first, unplug the system. then press and hold the power button for 10~15 secs. Then plug it in and see if it will boot.
If not the next step will be to repeat the previous but before you turn it back on unplug all external components, except the keyboard mouse and monitor.
It is also recommended that you cite the model of your Dell computer as well as the version of windows you are running.