Unity 5.5.2f1 crashes after creating new project or loading existing project

Edit: This was fixed by rebooting. I still have no idea what the root cause was.

I have the same (or a very similar) problem. This is in version 5.5.2f1 (3829d7f588f3) revision 3680727. Whenever I start Unity it 'freezes' after the load/new splash, without bringing up the editor (whether I try to load a project or create a new one).

There's four processes in my task manager (they've been running for an hour now, without any resolution):

 Unity Editor
 UnityHelper.exe  (x2 - it seems to always spin up two copies of this)
 Unity Shader Compiler

I'm not running anything weird to my knowledge (no AV or firewall other than Windows in-built stuff).

The last lines of the editor.log are (after about 1100 lines of output):

 Determining assets that need to be reimported for target platform ... 0.036390 seconds.
 
 Hashing assets ... 0.007 seconds
   file open: 0.001 seconds (54 files)
   file read: 0.001 seconds (5.375 MB)
   hash: 0.006 seconds
 Refreshing native plugins compatible for Editor in 0.22 ms, found 2 plugins.
 Preloading 1 native plugins for Editor in 0.04 ms.
 
 ----- Total AssetImport time: 0.044338s, AssetImport time: 0.000000s, Asset hashing: 0.000000s [0 B, 0.000000 mb/s]

Does this make sense to anyone? Thanks to anyone who can offer any suggestions!