Unity is pretty bad when it comes to fonts it seems. Adobe products will tell me I am missing a font and even what that font is. Unity just says missing font with no explanation as to what that font was. Have a project from a year ago - machine was rebuilt so all fonts gone. Have a folder of fonts on the network… but as unity gives no clue as to what fonts are missing what the hell to do?
I find this a glaring omission.
Hopefully it’s me…
What is the specific issue?
If something needs a font you are required to specify it. To specify it, you have to point to it in the Project Hierarchy and it is bundled in the Project Assets so if a font is missing then it’s an issue with your project backup.
OK, so it is me. That’s good. 
I do have several fonts in assets/fonts so I’m not sure why others wouldn’t be in there. I have some text elements, like my high scores fields, where the font just says missing font and I don’t know what font to use. It’s for a client, so I need to use the right font is all.
It’s possible they got deleted accidentally because they were placed in some random folder like “/Stuff Rcvd 5-18-16/” or something. Either way all you have to do is drag them into the project hierarchy like every other asset, then point your Text assets to the font you want them to use. As long as you don’t delete them or destroy their serialization/metadata, they are going to work fine.