while using audacity to edit a song ripped from youtube. I cut the song to lenght i wanted using audacuty and faded out the end of the song.
When i was satisfied with the changes i made i tried to export the song but it would stay in mono format rather than sterio despite the original file been sterio. Help please

How do you know it was originally stereo?
How do you know it is now mono?
How (exactly) did you export it (including what settings you used)?

When you were recording did you see two sets of wiggly blue waveforms?

See: http://manual.audacityteam.org/o/man/audacity_waveform.html