Re: I just bought The Sims 4 Luxury Party Game Pack...

Follow these steps:
- Go to the 'my games' tab in Origin.
- Hover over your 'The Sims 4'.
You'll see a little screen come up with an 'i' and a 'play/download/install' button.
- Click on the 'i', you'll be taken to a different screen.
- Scroll a bit down and there you'll see all your expansions, game packs and stuff packs.
Check if you can start the download again through there. 
Or try right clicking the game in Origin, and choose 'repair game'.