Try restarting the phone. (Turn it off and back on). Sometimes that is all you need.
Should that not work then try going into "Settings" then "Apps". Locate the "File Manager" app and tap "Clear Data". This should not remove any files on the device, but only clear the info for the app.
You may, however, want to back up files before doing so, just to be safe. Back them up to a google account, computer, etc.