Again, revisit the reasons why you bought the company in the first place. Stick to basics and ask yourself: How healthy are the earnings? What are the growth forecasts? How likely is the company to achieve those forecasts? Did you miss something in your analysis? And even if you conclude that the market is definitely wrong, how long will it take to get it right? Can you identify any key trigger events for that to happen? And can you afford to wait?
Stock prices venture far away from reality in both directions and if you paid a fair price for the company and the earnings are coming through, or are likely to, there's every chance its performance will pick up.
The risk is that the company may be overtaken by subsequent events before the share price has time to recover - so obviously you'll need to watch closely any stock that's going backwards.
Selling stocks that have fallen in value often means that you buy high and sell low - no way to make a profit. However, just as it's unwise to sell stocks solely because they have fallen in value, it's even more common for investors to get it wrong in the other direction altogether.