If your Dropbox indicator applet icon suddenly has gone missing from the tray in Ubuntu, it likely means that Dropbox simply isn't running. You can however easily run Dropbox from your Dash Home – perhaps something many new Users of Ubuntu don't give much thought.
The most likely time where you will notice that Dropbox is missing from tray, is after a system reboot – 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, we recommend a re-install. If you find another solution, feel free to post it here in the comments.
The Dropbox Preferences
One reason why Dropbox doesn't show up in tray on startup, may be that someone has unchecked the Start Dropbox on system startup option in the Dropbox preferences, so you should be sure to check if that is the case. You can locate the peferences of your Dropbox installasation by right-clicking the icon in tray, after launching Dropbox manually.
If it continiue 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