I saved a project and exited out as I had other work I needed to do, and I was playing around with different effects on my vocals. Didn’t realize I had left the effect on the vocals (and I habitually save after every action I take due to crashes in programs, Audacity never crashes for me though LOL) and when I came back it wouldn’t let me undo that effect. Is there any way to undo it at this point? Or to find a previous save file? Or at least the original vocals? Thanks 
Audacity does not save the Undo history, so you can only go back as far as when you opened the project.
A good backup strategy is to periodically use “Save As” and give the project a new, unique name, so that after a lot of editing you have a series of files:
MyProject001
MyProject002
MyProject003
…
Another good backup strategy is to always export recordings straight away as WAV files - if something goes wrong, you will still have the raw recordings to fall back on.