First of all, check if your desktop notifications are enabled in your account settings. Then, check the settings of your browser and see if it allows notifications to be shown.
- If you use Firefox:
Press Ctrl-I on Windows or Cmd-I on a Mac. This opens a window with a Permissions tab. Scroll down to the very bottom until you see Show Notifications and see if they are enabled.
- If you use Chrome:
Go to your Settings. Then click on Content Settings. Then scroll down until you see the Notifications section and see if they are enabled.