I’m not an expert, so take this with a grain of salt…
Before you try anything, make sure you have safety backup copies of the project made and stored elsewhere (other drive, USB thumb drive, DropBox, CD/DVD, …).
In the Scrivener manual (Help > Scrivener Manual), see
7.8 Backing Up Your Work
for a discussion of how backups work and where they go.
PROPER RECOVERY FROM PROJECT BACKUP COPY
You may find that a previous backup exists, which you can open in a second session of Scrivener and then retrieve the document via dragging it from one session’s binder to the other session’s binder. See 7.8.4, especially the “if you find you need to only restore pieces of a project” discussion near the end.
CRUDE RECOVERY
Make a copy (copy/paste) of the RTF file whose contents you want to get back into the Scrivener project. Make sure the copy is located somewhere other than in the Scrivener project folder.
Open the copy with something that can read RTF, presumably WordPad.
Select and copy the desired text (so that, behind the scenes, it is in the Windows clipboard).
In Scrivener, add/create a new text document for this text to go into, then paste (i.e. from the clipboard) into it. You can try Paste… and may or may not get useful or problematic formatting along with the basic text. Or can try Paste and Match Style version of paste.
If the above results in problematic formatting or characters, try adding an additional step… select/copy from WordPad, start and paste into NotePad (which should strip formatting), reselect and copy from NotePad, then paste into Scrivener via Paste or Paste and Match Style versions of paste.
LOOKING TO THE FUTURE
You may want to increase the number of backup copies that Scrivener maintains for projects.
You may also want to look into using the Snapshots feature for maintaining multiple versions of selected text documents, within a given instance of a project.
See 15.6 Using Snapshots in the Scrivener manual.
My naive guess is that if you can created a few snapshots of the chapter, you would have been able to revert to latest snapshot of that particular chapter.
And of course make sure that you regularly backup projects, probably via File > Back Up > BackUp To, to other/external drives or locations.
Hope that helps.
ADDITIONAL NOTE
In addition to use of Snapshots (see above), there’s another possibility for limiting the scope of this sort of loss… having chapters consist of a folder containing several text documents rather than consisting of just a single text document. That way, would presumably only lose a portion of a chapter, rather than the entire chapter. To view/edit the chapter as a whole, would select the folder in the binder and view/edit its documents via scrivenings (composite) view. Such multiple text documents per chapter could either be created when starting a chapter (from outline or for typical number of points in chapter arc structure) and/or by starting a first text document for the chapter, then splitting it periodically as it grows.