Sorry if this was already asked in a previous thread..

I'm playing through Downfall (the original version), and at certain points in the game, it just won't let me use items, I click on them but nothing happens.

In this specific situation, I'm in the basement and I need to put the brain in the guy's head, but it won't let me select the brain item, or any other item for that matter.
Has this ever happened to anyone else? Is my filed just completely screwed now?

That's a bug but it is passable - I think you need to click on the inventory arrows when you load the game. It gets blocked when you load up the game, clicking an inventory arrow unblocks it. "Just click on the arrow that scrolls inventory and voila."

Please note the 2009 edition has its problems & is unsupported now, hence we built a remake. But this issue you're facing is the most common one experienced with 2009 edition, and should be as simple as mentioned above to fix.

Thank you! Just like you said, click on the inventory scroll and voila, problem solved!
I'm so happy I can finally keep playing!