Hi Alan,
Thanks for letting us know.
You don't say which version of software you were trying to update from and to or the update method you were using. Please let us know as this might be significant to what occurred.
You were lucky - although the 'unpack' phase of an update should only take 5-10 minutes at most, it can take longer. Powering down or rebooting during this process can easily corrupt the microSD card, which then requires a full overwrite reformat of the card and reload of the software from scratch. The trick is to wait until the WiFi Hotspot reappears on your tablet or phone, or (as Steve has said) where there is a suspicion that all hasn't gone smoothly (such as if the WiFi Hotspot doesn't reappear), wait 20minutes or so, then connect the unit via its HDMI port to a monitor or TV before rebooting it. This allows the progress to be monitored to the point where the software reaches its 'login prompt', which identifies that the update or install has been successful. (You don't actually 'log in' but at that point can remove the HDMI / monitor.)
Best Regards
Peter