There are a few ways this can happen(probably all of which have hit me, so sorry for the long reply to get it all in)
1-was ANY other drive connected while you were working? Even a USB thumb drive? No matter if you are certain that you were not recording to that, if another drive was connected, re-connect that drive and cross your fingers.
2-I had sessions on a fairly new SSD(less than 6 months old) and it decided to "lose" files, with the most recent going first. After much cursing and bargaining with the client, I removed the drive and sent to Samsung for replacement. They upgraded the firmware and sent it back. Can't say I will ever trust that particular drive again
3-I was moving sessions between 2 systems(main studio and midi suite). As I went to finalize mixes, various files were disappearing(once again, the most recent, for some reason). I backed everything up(a lesson you need to learn right now) and talked the artist into coming back in for some fixes(and I ate the extra studio time) and we got the project wrapped up(and backed up 2 more times). Then I discovered the cause: my partner that was doing the midi work had the drive in a sled(so its a raw spinner hard drive, sitting in a tray). He would set the drive sled on the top of his computer tower and every time he took his headphones off, he set them down right on top of the drive
. What do headphones have in both earcups? Magnets
4-you can expand the Clips bin to show the complete file path, so showing that just might reveal where your missing audio SHOULD be.
5-for any portable drive, its possible that it will come up with a different drive letter from time to time(especially if you often connect external drives). If the drive letter gets changed for that(or any other) reason, Pro Tools will not be able to find audio on that drive without some user intervention(changing the drive letter back is the best option to me).
6-If you do manage to find your missing data, do a Save Copy>check the box to include all audio files>save this copy on another drive(or two) as backup
