When a Salesforce sandbox org is refreshed, your Salesforce user account is sometimes deleted and recreated.
Alongside this, Salesforce generates a new org ID for that sandbox as detailed here
The issue:
After the sandbox refresh completes, your account on that sandbox may no longer exist, or if your account does exist you will no longer be able to connect to that sandbox using the same connection as the org's ID has now changed. This means that it is seen as a new org by Gearset, and any authentication tokens or sessions to that sandbox will no longer be valid, and any org connections to that sandbox will no longer work.
Using your sandbox for Gearset login authentication:
If you use the sandbox as your login authentication for Gearset you'll need to change your login as per our guide here.
You'll lose access to your previous history in Gearset, as your Gearset account is tied to a specific Salesforce user.
Due to this we recommend you don't use sandboxes for paid Gearset subscriptions as noted in this doc.
Using your sandbox as an org connection:
If you have your sandbox as an org connection in Gearset, you'll need to remove that connection, and then re-add the connection to that sandbox.
Re-authorizing the connection won't work, as the orgs ID has changed. There is further information on this here.
Incorrect username and password after re-adding your sandbox org:
If you do re-add the org connection and receive the error of incorrect username or password please see this doc on the steps to take to resolve this issue.