It sounds an awful lot like the flash-memory problem I experienced
late last year. The scenario is this: your batteries are low, but
not so low that you get a battery warning; flash takes quite a bit
of current when you write data; a flash write takes enough current
to cause the zoomer to suffer reduced voltage momentarily, causing
some kind of run-time error and a "reboot" message, probably corrupting
the open file at the same time; when you reboot, you have a corrupted
file.
The fix: get a new flash driver from SunDisk. I think it may have been
posted to zoomer-list earlier this year, and it might be on an ftp
site or AOL somewhere.
If you're notebook file wasn't residing on a flash device, then I don't
know what the problem/solution might be.
-Brian