I had a R script open in RStudio. The file was saved many times over the course of several weeks and worked perfectly fine when RStudio was opened and closed. However, today
It might be an old thread and it might have been covered in 'user4458796' answer in suggestion #1 ("Use the history..."), but:
My friend had the same problem and we managed to recover the code from a 'history_database' files located on Windows at: 'C:\Users\%user%\AppData\Local\RStudio-Desktop\'
I assume there is an equivalent location in Linux in general.
Hopefully I won't get downvoted, just sharing my 2cents.
Ben.