Mac mini unable to start from boot disk or installer memory stick

I was installing High Sierra OS from a memory stick onto my Mac mini (mid-2010, A1347) when the power failed. The disk was corrupted, and I had to erase to a clean disk.Now it's giving 3 short, 3 long, 3 short beeps on attempting to start up, and this seems to be saying 'attempting to download firmware'. Every time I attempt to start, from external boot disk or from installer memory stick, it shows a circle with a line across, where the Apple logo should be. Is there a formal way to do this, because it doesn't seem to be doing anything?