I deleted a model for a simple prefab but Unity won’t let me simply reimport it from the asset store. I can’t Ctrl+Z this, and I don’t know how to simply just bring the pack of assets back into my project and only import the thing I deleted.
Have you checked the system Recycle Bin, or Trash can for the deleted files? Usually it’s just to drag them back to the project Asset folder with the file browser.
Upon importing a package you can choose which files to reimport. Of course you need the package file first.
If you are on Windows you can look into %AppData%\Unity\Asset Store-5.x and see if you can find the download cache of the package. If you reimport it manually you can pick which files to reimport. The GUIDs may change so you might have to rewire it.
Also, this is THE opportunity to think about adding version control to your project 
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