Mathematica 10.0.x freezes in Initializing kernels

Yes, there was a recently pushed incorrect paclet update that will cause this startup hang. All platforms can be affected, not just Windows.

For a workaround, start a standalone kernel (WolframKernel.exe on Windows, WolframKernel in a terminal on Linux; on Mac you will need the full path to the kernel binary, typically a location like /Applications/Mathematica.app/Contents/MacOS/WolframKernel) and evaluate

PacletSiteUpdate /@ PacletSites[]

PacletUpdate["CloudObject"]

which should allow a normal startup afterwards.

It is also possible to

• disable your computer's network connection
• start Mathematica
• turn on the network connection

• evaluate

  PacletSiteUpdate /@ PacletSites[]

  PacletUpdate["CloudObject"]

after which things should work again, even if the network connection is left enabled.

Update

As of this edit, the broken paclet is no longer on the server. For those still experiencing the startup hang, the steps above still work, but the easiest fix would be to either delete the entire Paclets folder or just the pacletSiteData_10.pmd2 file, which is located in the user base directory, typically under

• C:\Users\<username>\AppData\Roaming\Mathematica\Paclets\Configuration on Windows
• ~/.Mathematica/Paclets/Configuration on Linux
• ~/Library/Mathematica/Paclets/Configuration on Mac OS X

I'm adding an alternate solution for Mac, as the suggested answer did not work for me. It might be due to me force quitting it prior to looking for a solution.

This method is a bit of a scorched earth process though.

In Finder go to ~/Library/Mathematica. Once there delete the Paclets folder.

Next start up Mathematica again. Create a blank worksheet. Then evaluate PacletUpdate["CloudObject"].

Wait for the Out response before quitting Mathematica.