My 2 year old iphone 4 was around 38% battery when I closed my eyes. (TV nap)
When I opened my eyes, the apple logo was showing. It repeatedly reboots. If I use a wall power adapter I can get logged in and look around for maybe a minute before it crashes and starts over.
While it is up, no phone service is available. The PHONE icon has a huge red dot on it.
Online support database suggests charging it.
But the battery shows as 100%, plugged in.
If I remove the power cable, it immediately dies and is non-responsive.
I attempted to restore.
It fails with Unknown Error 21.
Now it is a very expensive brick.
Retrieving data ...