Can't access BIOS setup, Inspiron 660s, Windows 8.1

I recently installed Ubuntu on a USB flash drive so I can boot from that when I don't want to run Windows. However, I cannot get into the BIOS setup to change the boot sequence to make the computer boot from the USB drive. When I turn on the computer, no Dell logo appears before Windows, and no matter how many times I press the F2 key after turning the computer on, the BIOS setup screen does not appear. I remember that I could get into the BIOS setup when Windows 8 was installed, but not since I upgraded to Windows 8.1. After I press the F2 key, the computer appears to hang and the screen remains dark, so I have to turn it off after a while and nothing happens.

I tried disabling Windows fast boot. I tried holding down the Shift key while clicking on Restart, and I get a menu in which I can select to "use" the USB flash drive, but when I click on that, the computer restarts, does not boot from the USB drive, and appears to hang. There does not seem to be any way I can get into the BIOS setup.