Hi,
What should I do? I'm guessing I can delete the block data and start over downloading the whole thing.
This should take me, well, forever. Anything else to do?
And why did this happen in the first place?
Win+R: explorer %APPDATA%\Bitcoin
don't kill wallet.dat (or buckup file move here):
delete all files in %APPDATA%\Bitcoin\
+ in <blocks>, <chainstate>, <database>
don't delete folder <index> inside, empty this.