Dropbox users were unable to access the website and mobile apps over the weekend, which the company blamed on a glitch in its server upgrade process.
The file sharing company denied hacking claims, calling them being 'a hoax', after several Twitter accounts of hacker groups claimed responsibility for disabling the services.
"This was caused during routine internal maintenance, and was not caused by external factors," it said in a statement.
"We are working to fix this as soon as possible. We apologise for the inconvenience."
Earlier reports said hacker group "The 1775 Sec" had claimed on Twitter that the move was carried out in memory of internet activist Aaron Swartz who committed suicide and died on the same day last year.
The storage firm's latest status update noted: "Dropbox should now be up and running for all of you, but we're working through a few last issues with the Dropbox photos tab."
Last November, the cloud storage firm sought $250m funding which would boost its value to $8bn.