What are reasons for 401 Unauthorized (API Error)?

[Ilya_z]

I am an app developer for Shopify for 3 years. Our company uses offline api tokens, so they should be permanent.
Few days ago I got a ticket about shop where app stopped working. I checked the issue: 401 Unauthorized api error occured. So, an api token has become invalid. I checked a shop domain - shop was working. Why could the token become invalid? What are reasons for it?