Well, if you have different clouds which are already used with Boxcryptor, but you don't know where/which ones (however, at least their a) email-address and b) should be known), then:
- Check via the "Locations" tab in Boxcryptor where the sync folder is located and what/which clouds are set up there.
- Open the settings of that specific cloud software and see which email-address you have. Sign in into your cloud.
Regarding your second question: if one cloud/provider goes offline or closes, then you will see your files as long they are not deleted in the cloud (and deleted on your local devices via their sync).
For sure, in such a case you will be (hopefully) informed before they close the doors.
To avoid any data loss: do a backup of your (cloud) files.