Syncing won't happen over poor or disabled internet connections. That's why you need to check the status of your Internet connection. Click on the WLAN or LAN taskbar icon to view the status of your Internet connection. If your are experiencing a poor internet connection simply turn off all downloads and avoid browsing the internet until the syncing process is completed. Based on your current requirements, you can prioritize the upload speed over download or vice versa. Learn more: How to limit the download or upload speeds in Dropbox.
Check if synchronization is enabled
Make sure synchronization is enabled in Dropbox. Click on the Dropbox taskbar icon Settings > Resume Syncing.
Renew your paid subscription
You should ask yourself the following questions:
Have you downgraded your account recently?
Are you using a pro account? If yes, have you renewed your monthly or yearly subscription?
Did you benefit from extra storage through promotional offers? If yes, has the promotional offer expired?
When a paid account or promotional offer expires, Dropbox automatically switches to a basic account. If your online data exceeds the storage limitation of a basic account, you won't be able to sync new files. Upgrading your account will fix this issue. Right-click on the Dropbox taskbar icon > Upgrade.
Errors with the Dropbox client
The Dropbox client has encountered an error and won't sync your files. Try to restart the application by clicking on the taskbar icon > Settings (Cog icon) > Exit Dropbox. Launch the Dropbox app again and resume syncing.
You can also access your Dropbox cloud storage via your web browser, by connecting to the www.dropbox.com. Files and folders can be viewed, edited or downloaded using the web interface.