Why did all of my files disappear?

As I am a basic user, the policy of dropbox says I can only recover the files deleted within 30days as they will be deleted for real!!! Can someone tell me whether this statement is really true or it is just bullshit and I might have chances to let the staff of dropbox to recover them?

That is correct. Basic accounts, or Pro accounts without the Extended Version History addon, can only recover files for 30 days from the date they were deleted. Beyond that they cannot be recovered, even by Dropbox staff.

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rjothvas wrote:

... a Basic account that I haven't used in awhile ...

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How long is a while? If an account remains idle for more than a year, Dropbox will remove it, which also removes all files from the system.

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... and now it's telling mee that it can't find any of my folders, and dropbox made me use Installer

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Is it possible that you started a new account, instead of signing into your old one? If the old one had been removed and you installed with the same email address, you would have started a new, empty account.

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are my files now all gone?  If not, where are they?

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If you're signing in with the same email address and the account is empty, then it's likely as I described above. Otherwise you can try checking the Events page and Deleted Files to see if the files are there, or see if it lists what happened. If both pages are empty (aside from anything new you've done), then you're in a new account.