Monday's dramatic morning market plunge is enough to unsettle even the most steeled investor. But for some, the morning was made even more stressful by glitches reported at a number of major online brokerages.

Fidelity Investment, Charles Schwab (NYSE:SCHW),and Schwab acquisition target TD Ameritrade (NASDAQ:AMTD) all reported delays and outages on Monday morning due to higher-than-usual volumes. Fidelity's issues appear to have been particularly significant, with customers on Twitter posting screen shots showing their accounts zeroed out.

A series of lines and data representing a flurry of Wall Street stock quotes.

Image source: Getty Images.

Fidelity and Schwab officials told reporters that their systems are up and running, but clients might experience delays or technical issues because of the amount of people coming to their sites. TDAmeritrade took to Twitter to apologize for what it called "order fill messaging delays."

Online brokerage Robinhood also appeared to have issues around the time markets opened on Monday, according to DownDetector.com.

Market commentators including Motley Fool co-founder David Gardner often preach the importance of remaining calm during a sell-off, but it is also important for stock investors to feel in control as they are watching markets dive lower. The S&P 500 was down more than 100 points, or 3.15%, at midday on Monday, one of its largest single day declines in recent memory.

Perhaps the best advice on a down day is to avoid looking at your portfolio, but the glitches that made that decision for brokerage customers only added to the panic feel to Monday's trading.