I've lost one of my folders after upgrading my Dropbox account.

Glad you were able to find it, taalib! 

Since the folder in question is 5TB, is it safe to assume that you were previously part of a Dropbox team and this folder was unshared? 

If yes, and since the folder is way too large for your current quota (which, if you’re currently on the Professional plan, should be 3TB), you’d need to re-upgrade to the Dropbox team plan, so that you may have enough space to add it back to your Dropbox account.

NancyI have checked Deleted files and Events pages,nothing is there, In shared folder it is showing me 5 TB Not enough space to access folder

Hey taalib, thanks for your post. 

In general, upgrading your Dropbox account doesn’t affect the files you’ve stored there. However, let’s have a look into this to see what may have happened.

Have you already checked your Deleted files and Events pages?

How about your Sharing tab? Was the folder that you’re now missing shared with anyone?

Let me know and we'll go from there.