Have you swiped completely through the icons to find it? There is no other settings that I am aware of that would restrict that. Have you recently updated dropbox? If you have, and you are still running iOS 7.1.12 as your tags states, that could have something to do with it (shouldn't, but could).
You could also try a reset: Simultaneously hold down the Home and On buttons until the device shuts down. Ignore the off slider if it appears. Once shut down is complete, if it doesn't restart on it own, turn the device back on using the On button. In some cases it also helps to double click the Home button and close all apps BEFORE doing the reset.