You need to have a look at the Windows Cluster manager for the Windows servers that SQL is running on. As you may know 'Always On' relies on a cluster created using Windows Failover Clustering.
You should be able to find information online about how to access that on the version of Windows you are using. If you can't find the information then post the information and someone should be able to advise.
It should show you the state of the cluster, the items included in it (File Share Witness e.t.c. as the previous reply mentioned) and any errors as well. If you can let us know what's there we can try and advise.