A couple of thoughts as to why there is no sound.
On most DAW's (I have no experience with Cubase) you have to tell your program in the preferences section where to look for your audio input. If your tascam is plugged in to the USB port, it should be listed as a choice. Select that if you haven't done that already.
I assume you are playing a guitar with a pick-up going directly in to the line in, with the tascam set on guitar. If you are micing the guitar and going in to the XLR input, you need phantom power on to your mic if it's a condenser mic, and the tascam should be set to mic. Which way are you recording the guitar? Then all you need to do is create a guitar track in Cubase, arm it for recording and hit record. If you've done that, are you getting a waveform in Cubase with no sound or nothing at all? If you are getting a waveform, the sound is getting in to Cubase and the problem is with your monitoring. It may be as simple as hitting the "monitor on" switch. Give us more info and we can probably help figure it out.
