Welcome to the community! Though sorry to hear you’re having trouble. Depending on the scale of the data you need to fix, your choices involve either making bulk changes on the web, or writing an API script to fix the changes for you.
I myself made a similar mistake with a status field in the past, wiping out a bunch of statuses in the process. I wrote this API script to fix the change which you could use as a starting point to reset values for your List field.