Can you Save the work as an Audacity Project? Nobody else can play those, but you won’t lose the work. You can keep opening the show until you figure out the export step. Audacity Projects have two parts, the AUP file and the _DATA folder. You need both and they have to be in the same location, folder or directory to open.
Audacity doesn’t do surveillance very well and this is one reason why. Extended length and compressed file formats have nasty surprises.
As I said to Dario on feedback@, he must be sure to use Audacity 2.0.6 or later if he is saving very long projects. Otherwise the project may reopen as silenced orphan files.
Dario if you need more help please be specific what format (WAV, MP3 …) you are exporting to and what the exact symptoms of the problem are. And please, tell us your Audacity version (see the pink panel at the top of the page).
If you want a lossless format, try FLAC which doesn’t have a size limit (as long as you are using 2.0.6 or later).