This is a common problem in Premiere Pro which can happen to any media file. You should be able to fix it like so:
- Quit Premiere Pro
- Open Premiere Pro and before doing anything else follow these steps:
Go to Premiere Pro's Preferences / Media Cache / Remove Media Cache Files
Then select "Delete all media cache files (requires restart)" and press OK
After performing these actions it should be possible to link the media files again.
In case this solution didn't help it is also possible that the media files you're trying to link are not fully synced to your hard drive. For example if you're using Dropbox or Google Drive, these services offer "cloud only" files, where the content of these files has to be synced to hard drive first. If your project folder is on this kind of cloud folder, make sure the files are fully synced to the hard drive (see manual for your cloud storage provider).
If this isn't the case, please contact us.