There are three things you can restore to the default settings: the keyboard shortcuts, the Sandbox layout and everything you have changed, including keyboard shortcuts and preferences. The latter will be discussed here.
If you only want to reset the keyboard shortcuts, check how to do this here.
If you only want to reset the Sandbox layout, click here.
Only do this when you can't start CRYENGINE Sandbox normally, because it resets all the settings you have customized.
If you need to perform a hard reset of the CRYENGINE Sandbox, you can delete all adjustments you've made by deleting the following folder on your hard-drive: C:\Users\<YOUR USERNAME>\AppData\Roaming\Crytek\CryENGINE.
The AppData folder is a hidden folder, so you may not be able to see it. Make sure hidden folders and files are shown by doing the following:
(Instead of step 5, you can also click the Options button and choose Change folder and search options. This will then give you the same window as mentioned before under Windows Vista/7, step 5.)