I started to record my gtr & in default to a stereo track - but the right meter only recorded. Then I wanted add more of the piece and the previous recording was deleted. When recording a gtr should it a mono track - if it can be a stereo track how do I set it so both meters will be receiving the sound - and how do I record other parts of the piece without the previous parts being deleted. 
Set the number of recording channels to “1 (mono)” http://manual.audacityteam.org/man/Device_Toolbar
I don’t know how you managed to lose the other track, but perhaps this will help: http://manual.audacityteam.org/manual/help/manual/man/audacity_projects.html