I have just opened a document that had several applied InDesign files and now all of the text shows as Basic Paragraph and [None] under character styles.
Is there a fix to get the styles to re-assign as they were?
It would be highly unusual for this happen all by itself. Are you sure you opened the right file?
No, there is no fix if this happened. Do the styles still exist? You will have to reapply them.
But first, open the Story Editor with the frame selected and confirm that the styles are really gone.
I agree with Bob that this is unlikely to have happened by itself. Check your Recents list, and check to see if it was closed without saving. But if the styles aren't applied, there is no quick fix. Sorry.