Hi Aisling,
I will add to what Simon said by saying that all is not lost if you can't find the YW5 file because, as he said, the actual content of your scenes are what is in the RTF files. Now, the thing is, they may be out of order, and that is what the YW5 would do, put the scenes in their appropriate chronological chapters for you (amongst other things).
If you can't find that YW5 file, well, just open up the first RTF file which will probably be RTF_00001, and read it. RTF_00001 will be the first ever scene you wrote in your story. This means if the first scene you ever wrote was a scene that happens in Chapter 15 half way through your story, you have some detective work to put all the contents of the RTF files in story order. If the first scene you wrote was the opening scene, then hey, you have found your opening scene. Either way, copy and paste the content of the RTF file into your word processor of choice, and then open RTF_00002 and read that one. It will be the second ever scene you wrote in your story. If you're lucky and you wrote the second scene of the story straight after you wrote the opening scene, now you have the opening two scenes of your story fixed. But if it was a scene towards the end of your story, just copy and paste the content into your word processor of choice again but leave a gap and maybe a note above as to approximately where the scene comes in the story, and then keep opening RTF files and pasting them into your word processor until you have opened them all and pasted them all in around about where they should go.
Hopefully you have most of the RTF files you ever created, or even all. So you see, your story is not lost without the YW5 file, it just needs a bit of putting back together.
KM