If your Dropbox indicator applet icon suddenly has gone missing from the tray in Ubuntu, it likely means that Dropbox simply is not running. You can however easily start Dropbox by clicking Dash, and searching for Dropbox.
Sometimes the Dropbox icon goes missing after rebooting – to launch Dropbox again, do a search for Dropbox in Dash Home, then simply click on the Dropbox icon when it shows up.
If Dropbox is starting when you startup your system, but only the icon is missing, you can try re-installing Dropbox.
The Dropbox Preferences
One reason why Dropbox does not show up in tray on startup, may be that the Start Dropbox on system startup option in the Dropbox preferences has been unchecked.
You can locate the preferences of your Dropbox installation by right-clicking the icon in tray, after launching Dropbox manually. Then simply check the option to make Dropbox start when you login.
If it continue to happen over and over every time you reboot, you may want to try and remove Dropbox entirely with the apt-get remove command in terminal. I.e.
sudo apt-get remove --purge nautilus-dropbox
The --purge part of the command will also remove the configuation files of the package that you are removing, which can be good if trying to solve annoying out-of-nowhere like problems such as this.
You can also remove your Dropbox directory – it shouldn't matter if the content has been saved to the cloud before uninstalling. To remove the Dropbox directory, simply use the rm command with the r option, which will make it remove directories and their contents recursively. I.e.
rm -rv ~/Dropbox
The rm command is used to remove a directory, the -r makes it a recursive remove, which basically means it will remove all content as well as the directory. The v just makes it explain what is being done.
To re-install Dropbox, just run the apt-get install command from terminal. I.e.
sudo apt-get install nautilus-dropbox