What it means
The status bar in the bottom-right corner shows 'No connection' when MetaTrader cannot talk to the trading server. Prices stop updating and you cannot place or manage trades until it reconnects.
The bars in that corner tell the story: red or grey means no link; green with numbers means you are connected and shows the signal quality.
Common causes
Your own internet is down or unstable. A firewall or antivirus is blocking MetaTrader. The wrong trading server is selected in the login. The server is temporarily down for maintenance. Or your session was logged out (for example because you logged in elsewhere with the same account).
Step-by-step fix
Check your internet works in a browser. Click the connection status bars and pick 'Rescan servers' or try re-logging in. Confirm you selected the correct server name from the list your broker gave you.
Allow MetaTrader through your firewall and antivirus. Restart the platform. If nothing works, restart your router. If it is still down, check whether the broker has announced maintenance.
How to prevent it
Use a reliable connection, and a VPS if you rely on trades running around the clock. Keep the platform allowed in your security software. Note your exact server name so you can reconnect quickly.
Broker's fault or yours?
Most of the time it is your local connection or settings. If your internet is clearly fine and other people using the same broker report outages, the server is the problem — a broker with frequent, unannounced downtime is a real red flag for anyone who needs reliability.