I have an old eMachines D3024 that I use to run Fedora Linux. This morning it got "stuck" on the BIOS screen while booting. I went through the usual behavior of reseating RAM (DIMM) and internal cables. That didn't help at all. Then, I noticed that the Microsoft Wireless Optical Mouse Receiver 2.0 (a USB mouse RF receiver) was not lit up when the PC was turned on. It normally lights up a small LED on the front of the receiver. So, I grabbed a wired mouse and plugged it in to the PC. And, yep, the PC booted with a functional mouse attached.
This is the first time I've see a PC that would not boot past the BIOS startup screen just because a mouse was not installed. So, it sure was a head scratcher problem for me.