|On a computer that age you must Enable Legusy USB devices in the BIOS in order to use a USB keyboard.|
If the keyboard previously worked then the CMOS battery is probably dead so you have lost your custom BIOS settings.
Now it is a catch 22. You need to use a PS/2 keyboard to get into the BIOS to make that change. Once you enter the BIOS you can tell right away if you have lost the custom settings. If the date and time are wrong then the BIOS has reverted to the default settings. You will need to replace the CMOS battery on the motherboard.
Until you can get one do not interrupt the power to the computer and the 120VAC will hold the settings. You can shut the computer off. Just don't use a power strip or shut off at the PCU switch.