Sounds like your user token expired.
Usually a user token is valid for around 4 hours.
So then you would use the refresh token to get a new access token and continue working.
When you got the user access token you will have got an expires in, which is in seconds. And that tells you how long is left on the token.
See also the validate endpoint to set a token
And refresh
The app access token doesn’t apply here too much.