How do you restart the download manager service? I was using my Nexus 7 (android 4.2.2) through the hotspot on my phone and the download manager was trying to download a large app update. I pressed and held the notification to get to the app info for download manager and disabled it. How do I re-enable it?
Go to Settings > Apps then scroll to the right.

You'll see a few tabs:


Downloaded
Running,
All
Disabled (if any of your apps are disabled)


In the Disabled tab find the Download Manager and enable it.