Write() vs Save()
I expect someone did something like the following.
write.csv(my_data_frame, file="/path/to/my/file.RData")
instead of doing
save(my_data_frame, file="/path/to/my/file.RData")
Overwritten?
The save workspace button of Rstudio should have created an RData file. If you're seeing that file is a csv file, then it was likely overwritten somehow. Unfortunately, that means the file that the workspace was saved to is no longer there. This could have by passing the .RData path to a subsequent call to write.csv() or perhaps when copying or moving files around from whatever file manager on the system is.
Recovery
If the file was indeed overwritten, recovering an overwritten file may be possible, but seems unlikly. It will dependent on your operating system and file system configuration, and is beyond the scope of this forum. I would not expect that the original RData file is recoverable.