This sometimes happens if your cell phone goes back to the lock screen.
Unlock the phone with your PIN while it is attached to the PC and you should be able to see the contents in the File Manager.
-PJ
On Android phones, sometimes you need to switch the mode on the phone also (from charging to file transfer or some similar setting).
I’ve also seen some USB-C cables that will only charge the phone when the connector is inserted in one direction, but if you reinsert the connector 180 degrees out from the way it was you will see file transfer options also. There are also some cables that aren’t functional for file transfer.