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
You will want to click on Disable offline files to bring the files and folders online. You will then restart your computer
If you go back into Sync center and back to Manage offline files it should look like the following screenshot
