If you accidentally added an entire drive - or a folder which contains a huge amount of files - as a custom VST folder, Reason 9.5/10 might stop responding during launch.
If this should happen, just delete the "Reason Plugin Folder.prf" file in the following location:
Windows:
%APPDATA%\Propellerhead Software\Reason
or
Mac OS:
~/Library/Preferences
The user (~) Library folder is hidden by default. To find it, select "Go To Folder…" from the "Go" menu in Finder, and paste
~/Library/Preferences
and click "Go"
Then, launch Reason 9.5/10. Now, only the default VST Plugin Folders should be scanned.