If you accidentally disliked a song on Spotify, you can find it by going to your Spotify Wrapped feature, which shows you your listening stats for the past year. You can find it by clicking on the gear icon and selecting account on the app or by going to the Spotify website and logging in to your account. Then, scroll down to the section that says Your Year in Music and look for a card that says Songs You Loved (And One You Didn't). You can see the song that you disliked and unhide it if you want.