I guess my Junk E-mail folder is broken; the icon of the folder holds a "blocked" symbol (red circle with a diagonal line through it) so it looks like it is disabled somehow.
Why did this happen and how can I enable it again?
To be clear; is the correct icon of the Junk E-mail folder. If it looks like that, it doesn’t mean that the Junk E-mail filter is broken.
If nothing is being filtered to the Junk E-mail folder, it often is the case that you have added your own address to the Safe Recipients list. All emails sent to that address will then be automatically marked as "not junk".
For instructions to remove yourself from the Safe Recipient list see; Junk Email still delivered in Inbox
If messages move to the Junk E-mail folder unexpectedly and you wonder what exactly moved it to it, see; Safe-listed mail still ends up in Junk E-mail folder