If you're having trouble finding your old gym when searching in the app this could be due to an expiration date set on your access to that gym.
Coaches have the ability to set a date when your access to their gym expires.
To re-gain this access, you will need to change your primary gym to something else like "CF Mainsite" and re-add your old gym. If your gym utilizes an access code, you will need to re-enter this to join.
You can change your primary gym by following the steps here.