I assume you're recording into an Instrument track with a VST and the VST is the sound you can hear.
There are a few uncertainties about the question:
- You are recording MIDI data and not audio, right?
- By saying you can hear while you play, are you referring to sounds coming from Cubase and not the keyboard's built-in speakers?
- When you say Cubase does not "display" the notes, but you can "see" that it is recorded, has Cubase created a MIDI event box?
Firstly, try the following:
- Disable Monitoring when playing back.
- Set an output routing in the inspector if you haven't yet.
Otherwise, did you follow all the steps as laid out in the Operation Manual? The following steps are listed (per the manual, not verbatim):
- Select MIDI input and output routing in the Inspector (done, as you heard the sound)
- Review MIDI channels and ports (done, probably)
- [Setup quantize, etc]
- Set Record enable
Now, if the notes played during recording are still missing, it might be a technical problem. Try the following:
- Enable MIDI thru (in Preferences)
- Reset the ASIO driver (by switching to something else, and then back again)
- Make sure all the clocks are happy and in sync (see MIDI in Device Setup: "Use system timestamp...")
If the recorded event is visible on the track, then it is an output problem. Import an audio file into another track (audio track) and see if you can hear that. If that doesn't work, check device settings (ASIO driver...) as well as VST Connections (press F4).
In general, read the Operation Manual. Cubase's can be especially daunting, but even if it doesn't solve your problem, there are lots of other tricks hiding in there.