The credentials used by Nelio Content to access any of your social profiles may expire depending on the social network. In that case, Nelio has no access to such profile and all its scheduled messages would fail.
The Social Profiles tab you find on Settings shows the Profiles connected to Nelio Content. If any of them needs to be re-authenticated, that social profile will appear blurred and will include the Re-authenticate button at the right of the social profile.
Click Re-Authenticate and follow the same process described in connect social profiles to give access to Nelio Content again.
In addition to it, if you want to refresh the credentials of the profile (it may be helpful to LinkedIn profile credentials, that will expire after 60 days from the renewal), you can use the Refresh link that appears when you hover over each valid profile.