Mathematica 10 Freezes after update

Updated: 
Tue, 2015-09-08 09:52
Issue Summary: 
Mathematica 10.0.x freezes on "Initializing Kernels"
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
 
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 PacletUpdate["CloudObject"]
after which things should work again, even if the network connection is left enabled.