Welcome to the forums.
I think that one of three things has gone wrong:
# The RPP file is corrupted.
# Reaper's configuration is corrupted.
# A VST in the project is buggy.
Obviously, there is the fourth possibility that your PC is goosed. Probably not.
A few things to try:
# File > Open project > New Song.... but before you click "Open," check the box that says "Open with FX offline (recovery mode)."
The project will open, or it won't. If it does, please report back before proceeding further.
# Consider resetting Reaper's configuration to factory defaults (if you're using Windows, it's in your Start menu under "Reaper"). You would then have to re-enter any custom VST paths, etc.
# If that doesn't fix the problem, upload the RPP project file (just the file, not the audio) to somewhere (eg a Dropbox or The Stash:
https://stash.reaper.fm/ ), post a link to that RPP file, and we the peeps will see if we can load it.
It might be useful to know what OS you're using.