Keyboard not responding at start up

Since you are currently stuck and need to get into the BIOS -one thing you could try is clearing the CMOS either by placing a jumper on the CLEAR CMOS jumper (look in your MOBO manual for the jumper location) or removing the CMOS battery and then pressing the power button with the system unplugged from the wall (this will drain any residual power from the system and should clear the CMOS ) -- Doing this should restore the CMOS to default settings and might get your ps2 keyboard working ( right now it probably has been disabled in the BIOS ) - IF that still does not do it try the USB keyboard in diffferent USB locations (could be some of the USB ports only load after booting while others may work before that so try plugging in to different locations.)

In older computers you had the ps/2 connections for both the keyboard and mouse at the top of the rear I/O panel and with the widespread use of the usb now some computer don't even have one but if your motherboard has the two color one then it will acept either a mouse or keyboard and right beside that ps/2 connector is usually two usb ports and I believe that these two usb ports are loaded first for a keyboard or mouse to be connected there for use in the bios. So try the usb ports at the very top of the I/O ppanel in the rear of the computer.
Also when you see that first screen that's when you want to press the delete key to enter the bios , because once that first screen is gone there are no options for the keyboard to be used anyways.