I use FontExplorer X for my font handling and after updating Illustrator from 25.0 to 25.0.1 opening old files drops font associations. I lost Avant Garde and the program could only find the ITC Avant Garde version, automatically replacing my text in the document (with wrong kerning, thus messing up all designs).
When looking for Avant Garde in the replacement lists, it couldn't be found. I downgraded illustrator to 25.0 again, and it works again... so the update messes something up with font handling big time.
So sorry to hear about this trouble. Could you please confirm which OS and version of Illustrator is this? Do you see the font installed on your machine? Are these fonts working fine in other applications?