After spending a very long time recording and editing a cover of a song on Audacity, I tried to export it but in doing so, every single track became a flatline. When I play back it is completely silent. Any export is also silent. Is there a fix for this? I spent a lot of time on this song.
Please see the pink panel at the top of the page and let us know that information.
It is better to save a project or export an interim mix earlier on, so you have somewhere to go back to. See http://manual.audacityteam.org/o/man/faq_opening_and_saving_files.html#backup.
If you look in Help > Show Log… there may be some information, but basically Audacity can no longer read whatever folder the audio data is in, or the data has been removed.
If you were still working in the Audacity temporary directory as specified at Edit > Preferences > Directories section, make sure Norton or other security or clean up software is not deleting files in that directory. File > Save Project As… to your Desktop or Documents before starting the recording can avoid problems with file clean up applications.