he or she managed to compromise my email account and dropbox.
As a general rule, it is not a good idea to keep unencrypted backups of your private keys in your email nor in a cloud service.
Hopefully this is not a lot of money to you, and you can let this be a lesson learned.
I don't really like the idea of keeping private keys in a cloud account, but if you insist on doing this, they should be encrypted, preferably with GPG, the private key of which is not in that cloud account. A safe deposit box is probably a better solution for an offsite backup.