It could be that there’s a third-party app blocking or hindering the syncing process. In this case, try temporarily disabling your antivirus, security program, or check that you haveconfigured your firewall to work with Dropbox - it might do the trick.
Also, could you by any chance be connected to a work network? If you are, check with your company’s IT department, since there may be an update on your firewall settings or anyother additional security measure placed which could be affecting your connection.
If all this comes to a dead-end, don’t hesitate to let me know and send me a screenshot(without any personal info) of what the Dropbox icon reads when you click on it and/or of any error messages that you’re seeing.