A Microsoft file hosting and synchronization service.
Dropbox doesn't suddenly stop loading, whatever that means, for no reason. Until you know the reason why Dropbox stopped loading, you won't know what needs to be done to solve the issue.
There are two ways to go: You can work with whatever sincere guesses you get here or come up with on your own, and hope that one of them actually solves the problem, without causing harm somewhere else.
Or you can contact Dropbox directly, explain to them exactly what you mean by "stopped loading" and have them explain what causes this to happen. Once you have the cause, you can find the solution.