You can always check the status of online services here Call of Duty®: Ghosts || Game Status .
If online services are fully operational and you are still unable to connect, you may want to try the following:
- Reboot your router.
- Try using a wired connection to sustain the connection quality.
- Establish a static IP address and port forward. Here are the port numbers you will need Activision Support .
If you have any questions on how to port forward, please contact your ISP or the router manufacturer for further assistance.