How do I UNBLOCK a site I erroneously BLOCKED, please?

in Firefox: Tools->Options->Content->Load Images automatically -> [Exceptions] button and remove any sites you do not want blocked for images.

I apparently blocked the site

I guess we should assume that was with Adblock Plus

As seen in Preferences ("Ctrl+Shift+F"), ABP maintains separate filter groups - Filters you created yourself (Exception Rules, Blocking Rules, Element Hiding Rules) - Separate Groups for each subscription filter

The Blockable Items Report ("Ctrl+Shift+V) is where you go to see what is blockable, what is blocked, and if blocked what the rule is by hovering on the row with the mouse. It will also give the source, so you can see if it was your rule. The right mouse button brings up a context menu with options. Columns I show are {address, filter, type, state} chosen at the right of the headers. I sort on State so block items in red are at the top.

You can use the Report to make the changes you need. Changes you make will be seen in the Preferences.

Thank you, that solution corrected my error. : )