If you use Console access for testing purposes, recent Java updates have presented a few problems to overall access to a VM.
Specifically, if you encounter any problems when trying to view a machine from a 64-bit OS, your web browser might not be detecting if you have Java installed. Windows 8 and 8.1 (64-bit) are mainly affected however other versions may be affected as well.
To resolve any instability, we suggest manually installation both 64-bit and 32-bit versions of Java from here.
There is also a known issue with the latest Java version (Version 7 Update 45) that causes a security prompt each time you connect to a VM via Console mode.
Clicking 'Don't Block' will display your VM. This will be resolved with the next version of Java.