I am using Cubase 5 right now. I also have focusrite scarlett solo audio interface. It all worked fine (i even recorded few songs) until today when i opened my project and cubase didnt detect my audio input. Yesterday it was fine, today something happened. When i open Guitar Rig I can hear the guitar, but not on Cubase. I will attach some pictures to the post. PLEASE help me 
I’m thinking that Cubase somehow lost the Scarlett’s VST input (and maybe output) bus connections. So go into the VST Connections menu and check them. This happened to me a few times (not recently though). I figured that somehow Cubase lost the ASIO connection to my audio interface but when I restarted Cubase it was again recognized (like yours seems to be) but the input and output buses (probably just inputs) in the VST Connections menu were lost. This didn’t happen to me enough to really figure out the reason but, if it keeps happening to you, check to make sure your USB cable is connected properly or try plugging into a different USB port. Could also happen if your computer is making the USB port go to sleep.
Regards. 