|OK, hopefully you unplugged the computers before removing the RAM. If the CMOS battery is weak or dead the CMOS will reset to default values. In most systems default values do not support legacy USB devices. That would mean that USB keyboards may not work in the start up screens. |
If you DIDN'T unplug both computers you may have damaged something.
Things to try on your computer. Re-seat all cards, cables, RAM. Re-seating is disconnect and reconnect the device.
If that doesn't help then try Clearing the CMOS. There are two methods to clear CMOS. Removing the motherboard battery for a period of time might be easiest for you.
The battery is a silver quarter sized coin shape battery. Take care when removing to note which side goes where and to not bend the contacts.
After leaving the battery out for a few minutes re-insert it and then start the computer.
Immediately enter the BIOS (setup screens. If you have successfully cleared the CMOS the date will be way off.
You will need to reset values in many screens. Default values may work fine for most screens. The time and date need to be corrected of course. The option mentioned above for legacy USB support need to be set to Enable.