At the point you are at, the computer is saying it can't find a disk with the OS in it.
To do a forced shutdown, press and hold the power button until the mini shuts down.
ensure you have connected to a Mac OS harddrive, or if you get the same result from the internal drive, then insert an OS X install DVD, at startup and hold the C key, until the screen displays. This boots from the Install DVD and allows you to look at the internal harddrive through Disk Utilitiy, or to reinstall the OS.
I have been a Windows user for decades. I was given a MAC Mini. I was in the middle of updating the operating system when it turned off. I turned it back on but all I get is a folder icon that has a question mark over it. i was told I need to reboot, but I do not know the key sequence using a windows keyboard. Any advice???