Try opening the Autosaves of the original Project:
Project Folder\Adobe Premiere Pro Auto-Save
Also, if you have exported any Sequences to Media Encoder via the
'Queue' option, Media Encoder will leave a truncated copy of the Project
that will contain the exported Sequence and its associated media here:
I really don't understand how this happened in the first place.
The only possible way it could have - operator error.
What version of PPRO are you running?
Does the content for the two projects have the same filenames (like C0000.ext, C0001.ext, etc.)?
Have you moved the content used in the first project?
At any point when you opened the first project, has PPRO prompted you to relink to the media?
While user error seems like the most likely scenario, if you're running PPRO CC (7.0), I'm thinking there's a possibility that Relink could have mapped to the wrong body of clips.
Even if my hypothesis is bogus, a solution could be to make all the content in the first project offline, then relink it to the correct content. If, that is, the sequence in the first project reflects your edits from that project rather than the second on, which is not clear to me from your post.