Steam had an issue a week or so ago, where downloads would get "stuck" and had to be cleared. Reinstalling Steam should have fixed that for you, but perhaps you will still need to nudge the stuck downloads to get things running again.
In the Steam Client, check the Downloads page. Is the download stuck there? Perhaps with a "Missing Downloaded Files" error?
If so, that happens on occasion, it's a corrupted temp download file. If you've got the Missing Downloaded Files error, try this:
1. Close down steam completely (so Steam > Exit in the upper left menu),
2. go to C:\Program Files (x86)\Steam\steamapps\workshop.
3. Delete the Downloads folder.
4. Then restart Steam. Resume the download if needed.
5. If that didn't help, hover over the "Missing Download Files" text and note the Steam ID. (the string of numbers).
6. Repeat step 1 and 2.
7. Open the 255710 folder.
8. Delete the folder with the mod's SteamID.
9. Repeat ...
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