If you have inadvertently set your folders and or files to offline mode you can do the following to bring them back.
Go to the control panel and open up the Sync center
The files will show up here if they are in offline mode.
Go to Manage offline files and make sure the settings match the settings in the following screenshot.
Make sure the button states the following.
"Enable offline files" if it does not click on it and restart the computer.