You probably recorded a mono signal on a stereo track. A STEREO track, by definition, contains two channels, a left and a right. Recording with one microphone, or one guitar, gets you a MONO signal. In other words, you get one channel for each signal source.

If you are using a single source, you need to set up Cubase to record a mono track. To do this, the Stereo switch button in the Inspector allows you to toggle between mono and stereo. The button will be gray and contain 1 circle if the track is set to mono, and be yellow with 2 circles when set to stereo. You can then choose you mono or stereo input from there.