What it means
'Market is closed' means the broker is not currently accepting orders on that symbol, even though you may see a recent price or a chart. Trading hours vary by instrument and by broker, and they are not the same as the underlying exchange's hours.
A last-known price on the chart does not mean live trading is available right now.
Common causes
It genuinely is outside trading hours for that symbol — many CFDs on shares and indices trade only during their exchange session, and forex closes over the weekend. A daily trading break (some instruments pause for an hour or so each day) can catch you out.
Public holidays shorten or cancel sessions. Your computer clock or timezone being wrong can also make you misjudge when a session opens. Occasionally the symbol is set to 'close only' or disabled by the broker.
Step-by-step fix
Right-click the symbol, open 'Specification', and check the exact trading sessions and any daily break. Compare them with the current server time shown in Market Watch, not your local time.
If it is a holiday, wait for the next session. If the hours look right and it still refuses, contact support to confirm the symbol is enabled for your account.
How to prevent it
Know the sessions of what you trade, especially share and index CFDs. Watch for holidays. Set pending orders to trigger when the market reopens rather than trying to force a trade during a closed period.
Broker's fault or yours?
Usually neither — it is simply the market being closed as scheduled. It becomes a concern only if a broker keeps a symbol closed during its normal session with no explanation, which is worth questioning.