first you have to start posting again using hashtags and all that stuff then you can use the follow/unfollow method to gain a few new followers that might interact wi your posts and share them with other and you can use tool for likes and comment exchange
You can come up with comeback posts/stories that will get your followers’ attention again. If you are still with the brand, you can do with the giveaways.
also, make sure to remove ghost followers before you start anything because they are bad for the account and make sure to post regularly and use different posts and hashtags and you can use some influencers for shoutouts nothing too big just some good account to mention you in their stories and posts.