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 can't 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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