Dropbox which is now among most trusted and popular cloud storage, experienced outage which affected users for ~48 hours. But dropbox explained in blog post what happened to the services and there is nothing to worry about.
On Friday their services went down because of some bug in a script that upgrades operating systems of their machines. Due to bug, OS of some active machines were also reinstalled and as a result Dropbox went down.
They said your files were never at risk because those machines whose OS got reinstalled were database servers and they use them for some features like photo album sharing, uploads and for API. They made this clear that outage was not due to some kind of hacking and now they are up and running like they were before.