Sound completely disappeared after exporting video

I spent hours to make a video only to find out that after I exported, the background song I used is completely gone. I went back to edit screen and works perfectly fine, so I have no idea what happened. I was using a 7 day trial and this was my last export (3/3) so I'm very disappointed that I won't be able to fix and export. The other two videos I made were perfectly fine and I didnt change any export settings either. Huge disappoinment.