But you were able to save it?
Three options I can see:
1. You buy FL Studio.
2. You send someone the file (is that allowed, though?), they open it up and convert it to MIDI for you, and you import it back into FL. You have to reassign all the samples, redo effects, stuff like that, but all the notes will remain intact.
3. You start over.
When I used the demo version, I had to export projects as MIDI every time I wanted to save. It's a pain to redo stuff when you open it again, but it works. Kind of.
It might be easier to work on the song in a free MIDI sequencer like Anvil Studio and then import it into FL only when the arrangement's done. I've done that for collabs with people who don't use FL.