OOokay, I reconnected the Kindle to my comp and
on the basis of this tip, I deleted a file called
USBnet.xml
that I found in the main root of the drive.
Then I disconnected and pressed down the power button for a long time to get the Kindle to reboot (it took a while/a few tries for this to work!) Now it works fine and the Collection Manager is working, too.
jwilker, hook your Kindle up to a computer and if there are any .XML files in the main root of the drive, MOVE them onto the computer (you know, just in case- safer than deleting!) then try powering down and rebooting the Kindle. It might work, and it would be safer/easier than redoing the whole thing.