If you just keep holding both buttons (might be 15 seconds or more) you should get a beep when it starts to boot. If it still has a blank screen after the boot, try touching the screen way down at the bottom edge with a fingernail. You should get a Windows CE start button. If you can get that, you should be able to get to the Rapid Deployment client from Programs to retry the OS update.
thank you very much Daniel! I was able to repeat the upgrading.