Should I hold on to the stock hoping that it will quickly rebound? Should I sell it and invest the proceeds in another instrument such as a mutual fund? An index fund may do better than a single...

Should I hold on to the stock hoping that it will quickly rebound? Should I sell it and invest the proceeds in another instrument such as a mutual fund? An index fund may do better than a single stock. What are other differences (such as risks and benefits) between owning a stock or even a small stock portfolio and investing in a broad (market) portfolio via a mutual fund?