The chart is frozen although you appear connected.
A feed glitch or a heavy template.
A chart redraws when ticks arrive, and each tick makes MT4 recalculate every indicator on that chart. If those calculations take longer than the interval between ticks the chart falls behind and appears to stop rather than to lag, because it never catches up. The chart clock stopping alongside the prices is what separates this from a genuinely quiet market.
The other cause is a stale symbol subscription after a reconnection. The connection to the broker is healthy and the status bar looks entirely normal, but the terminal has not properly re-established the feed for that one chart. That is why refreshing the chart or cycling the symbol fixes it without a restart being necessary.
Compare the chart against Market Watch. If the quote for that symbol is moving while the chart is not, the chart is the problem. After refreshing, the chart clock should advance and the current candle should extend within seconds during an active session. Checking a second chart tells you whether it is one symbol or the whole terminal.
Keep the number of open charts and loaded indicators to what you use, since all of them compete for processing on every tick. Be wary of downloaded templates, which often carry far more indicators than expected. Restarting the terminal occasionally is cheap insurance if you leave it running for days at a time.
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