I rotate a few Shuttle XPC cube desktop computers -- a few models. One of them was working fine last night, but this morning it would Stop | Start | Stop | Start right away during bootup attempts. Haven't run into that much, if at all. In my experience, the proprietary PSUs for these things just burn up one day, which you can obviously smell, and then it's dead, time for a new PSU: no intermediary failures like this. I looked around online, found some mention of cleaning out dust (that's not it here), flakey Power-ON button (doubtful), overheating CPU (doubtful), bad BIOS battery, and so forth. There were even some of those very common replies to similar issues saying "Dude, you've got a virus. Time for Combofix," etc. Bullshit ! If the HDD is temporarily disconnected, and you can't even reach the BIOS Settings screen, it ain't no virus ! We're talking pure, basic hardware issue here.
Google may be our friend, but it does dredge up a lot of very improbable advice.
I removed the 2032 button battery, which tested weak. Replaced it with a fresh one. No more Stop | Start on boot attempts, so it does not appear to have been the PSU, much as I surmised. But now there is no video coming up. Swapped in a brand new video card, but still no video. Can't be cables, connectors, or anything like that, as they remain a constant, and other computers here still come up normally, going in to the same monitor. So I guess that probably leaves the video card slot, of which there is only one PCIe on this computer, or some other general failure with the motherboard ? Unless I'm missing something here . . . ? I may even have a spare MB for this computer, but I was not really feeling up for a complete rebuild.