This can happen when Cubase SX/SL is being a ReWire host to Reason. It’s important that Cubase doesn’t release the ASIO drivers when it is run as ReWire host application.
To fix this, do the following:
1. Make Sure Reason is not running
2. Launch Cubase
3. In the Cubase menu: Devices-> Device Setup…-> VST Multitrack, Uncheck “Release ASIO Driver In Background”.
Reason should not freeze any longer when you launch it.
Note: It is important that you launch Cubase SX/SL and activate ReWire channels in Cubase before you attempt to launch Reason.