No bootable devices found after update

I installed RHEL 7.2 Desktop x86_64 on a Dell XPS. Installation appeared successful and I was able to reboot successfully. However, after yum update I am unable to reboot; I get "ERROR Verification failed (15) Access Denied" and subsequently "No bootable devices found". Dell diagnostic tests were all passed. Booting from the USB I am able to see the old installation directory.