On the Account tab, select Choose folders, check the boxes of the folders you want to sync. In the activity center, select More > Settings. This isn't common, but to fix it, go to your computer's system tray (if you use a Mac, go to the menu bar) and find the sync app.
Resynchronization does not download all files, it checks the changes and only syncs the updated files. Depending on the number of files synchronized, this may take some time. Note: Be aware that resetting OneDrive will resynchronize all OneDrive files. In the Run window, type: %localappdata%\Microsoft\OneDrive\onedrive.exe