The Nav unit should still continue trying to boot off CD, even if the installed software is now trashed.

So burn a new CD-R, using high quality media, and at the slowest possible speed as the current one obviously couldn't be read. I also recommend you use a different software version such as v27.0 or v28.0 as the update will not get applied if it appears that v27.1 is already installed.

If the CD won't eject, then leave for a few hours and try again, otherwise remove the power to the DVD/CD unit (blue cable) and press Eject repeatedly as you re-apply power.

Start all over again with the new CD. Don't turn the power off when it starts updating.

Good luck