I own a Thinkpad X121e and I upgraded its Ubuntu from 13.04 64bit to 13.10 64bit using the Update Manager. When the upgrade process suggested to restart the system, I restarted the computer and now it says "Operating system not found" after a very brief splash screen.

The splash screen says something like (too brief to see everything):

Intel UNDI, PXE-2.0 ...
For Atheros ethernet Controller ...

Check cable connection!
PXE-MOF: Exiting INTEL PXE ROM

I prepared a startup USB disk using Ubuntu 13.10. I tried the startup USB and it works very well on other computers. So I changed the booting order of X121e and tried all three USB slots, all said "Operating System not found" without any splash screen.

I also tried using startup USB with 12.04 LTS. Same results.

I do not really want to reinstall the system now. Any hints on how to proceed?