-snip- I get an error message "an internal error message has occurred, please check the log". I clicked "ok" and then the whole Bitcoin Core download just disappeared. I have no idea what to do now about anything.
I got a feeling that your Blockchain is set to be pruned and/or you don't have enough disk space for the blockchain and other data.
Because I can't find any other errors in Armory nor Bitcoin Core that will delete the already-downloaded blocks (
only if it's pruned).
If that's the case, you don't really have a choice but to export the prv key like the others have said.