Unless you have a Premium account, your files remain on our servers for 30 days until they are removed. However if your file is downloaded in full, it will remain on our servers for another 30 days. Therefore if your files are continually downloaded, they will remain active. If your files are important to you and you want to prevent them from being automatically removed then you should upgrade to a Premium account.
If you previously had a Premium account which has reverted back to a free account, then the normal storage limits for free accounts apply, and it is most likely that the majority of your files have been deleted.
Unfortunately if your files have been removed due to inactivity, it is not possible to recover them, even if you upgrade to a Premium account.
Premium members have unlimited file storage space, although a Fair Use Policy does apply if you exceed 5TB of storage.