Missing media. What has happened? Help😩

You C: drive is your main system drive isn't it? 

Either one of two things has happened:

• You've moved the source media folder to another place on your system
• You've changed the folder name or file names of your media

Either way, as Ann mentioned, you must relink the media. Either by the Locate button in the offline dialog box or by selecting the media in the project bin and right click + relink. 

Hard to tell what really happened.

Did you hit LOCATE to find your media?

Relinking offline media in Premiere Pro