So now I have this problem… Everytime I open Cubase 8 LE, I can play with my piano for like 5 minutes (sometimes less) and then suddenly stops working and to solve it I got to close Cubase, turn off and on again the piano and then open Cubase again.
The driver I’m using is Yamaha Steinberg USB, i’m using the latest version (this was recommended in the opening screen of Cubase)
My keyboard is a Casio Px-5S and it’s plugged in my PC. It’s also plugged in my Yamaha AG-06 with the Stereo L and R.
Is it possible that your computer power settings are set to save power by turning off certain USB ports. If not… try connecting to a different USB port.
Regards. 
I connected it to a different USB port and it seems to be solved now, thanks! But just to let you know, i’ve been using it for almost 1 year and I haven’t had a problem with that until now, why’s that? And also, I discovered that it stopped working whenever I pressed the keys very hard.
Thanks again 
You are welcome.
Tough to say… maybe a recent computer OS update? You don’t list it but, I assume you are running Windows. If Win 10 then we all know there have been a lot of updates that just might change settings and break things like you described.
Probably would be a good idea to check if you have the latest Yamaha/Steinberg driver installed. Maybe that would fix a few things too.
Regardless… I’m glad you got it running again. 
Regards