I assume you were doing that with the Audio Editor tool, and that you've clicked Apply several times and even left the tool and gone back to the main timeline.
As long as the project hasn't been closed, you should be able to click on Undo (or use the
Ctrl+Z keys to go back up to 50 steps. If you made more than 50 changes or have already closed the project, you're left with a couple choices.
One way is to simply drag the same clip from the media library onto the timeline. If you've trimmed the original clip, you'll need to match the starting & ending edges and select both clips and use the Sync by Audio tool so you can get everything perfectly aligned and then you can delete the modified clip and replace it with the unmodified version.
The other way is to go back to one of the autosaved versions of your project from before you started playing around with the audio. You can check how often the auto saves were made and see all the temporary projects from this screen: