This problem is caused by a faulty windows update.    USB devices may stop working after installing the February 13, 2018 update (KB4074588) I did and it worked for me.  Basically you have to use the media creation tool to create a bootable USB drive, boot your computer to the USB drive, enter the repair facility, get a CMD prompt and type in a long command to roll back the update that causes the problem.     
Hello, My Dell Desktop does not recognize my keyboard and mouse once I reach the Windows sign-in screen. Also, I am able to use my keyboard to enter the BIOS before the Windows sign-in screen. Once in the BIOS screen i can use the keyboard normally as well.   Please advise.
Windows Update KB4074588, break USB keyboard and mouse