Here are some possible reasons for a red "x" on the system tray or menu bar icon:
Your Dropbox is on a network device or removable hard drive with an inconsistent connection. If this is the case, restarting Dropbox may help. However, we recommend you keep your Dropbox location on the same hard drive as your operating system.
You are having difficulty establishing a secure connection. Correcting your system time or adjusting your proxy or firewall settings might help.
You're over your storage quota. You should make room in your Dropbox by removing files (or consider upgrading your storage quota). If you've been paying for a Pro subscription, check your account page to make sure your plan hasn't been downgraded. If you're a member of a Dropbox for Business account, ask your administrator to check the team's account status in the Admin Console.