How to fix a computer that says the signal is out of range- reset to 1920x1080 - 60hz?

I have a Dell Optiplex 780 which I believe has been modified to be better for gaming etc.
One day I plugged in my iPhone 5s without turning the computer on, after I did this the mouse showed up strange and the computer went black.
Apparently now the computer thinks my phone is the screen, so now it s unplugged it s telling me to reset to the settings I said above.
I have tried pressing F12, this brings up a menu. I think last time I clicked diagnostics it ran a scan and said nothing was wrong with the computer. This time I tried clicking system settings and it immediately went black.
Before I was able to access the computer and all my files for a few minutes, now I cannot do anything.
How do I fix this? Is it something I can do at home or do I need to send it back to the people I bought it from to be fixed?

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