Hopefully this was some kind of accident - ie Bio didn't get saved, rather than an intentional deletion.
You can find what the bio previously contained and that had been saved by clicking on the Changes tab across the top of that profile. On the change page it will show all the edits chronologically. If someone has made changes since your entries, it will show up here as well as your entries.
If that is the case, and you want to retrieve your hard work, you can cut and paste your entries from the changes page back into the bio. If something was changed, but accidentally not saved, it won't show up here, only the last saved ones will show.