In Robinson Crusoe, what happens when you lose an item that acts as an invention prerequisite?
There isn't a direct answer for this in the rules, but there are a couple of very closely-related cases:

On page 16, under Unexplored terrain type, the rules note that when you lose access to the last tile of a particular terrain type, you lose access to that terrain type and cannot build items that still require it, but you also do not lose any Items that you have already built.

A little further down, under Turning Items to Invention side, the only changes noted are that 


  If that item was required for building another Item or taking a specific Threat Action, you cannot do it now.


Combining those two sections, I reason that if you lose a prerequisite Item, you do not lose any Item you have already completed construction on as a result of that prerequisite Item. The main idea is that those prerequisite Items allow you to construct other Items; once you have constructed those other items, the prerequisite Item has no further function. 

Think of it this way: the Brick invention means you have a source of Bricks; losing the Brick invention does not mean that any existing items suddenly cease to exist. (Also consider another case: the Shovel allows you to build a Cellar. Does losing your Shovel mean that your Cellar disappears in a puff of logic? It certainly isn't made of shovels.)