Sorry i've been away all day....
To more or less answer your question, it's not unusual to see the charging circuit malfunction or die on the Pre3. You may be lucky and get full functionality back if it's just a component knocked loose, considering the Pre+/-/2 gives the same [?] error when the comm board is not properly connected.
That said, the Pre3 was redesigned from the ground up, so i have no idea if its bootloader also gives a battery error for non-battery conditions. You can take it apart and put it back together, taking care to properly reconnect everything. It might help, or not...
What i'll do now, is go back and take a look at the Pre3 teardown, if anything helpful can be gleaned from that. I'll be back.
Edit: ....here's the "teardown" Pre 3 Disassembly - a set on Flickr
I really hope that what you're experiencing is just a connection issue... in any case, your Memos are safe on the HP server, and files on the "USB" partition are not encrypted, so even in the absolute worst case, they can be recovered from the flash/SSD chip.