Typically this will be because you have changed your phone or been logging in with different Task accounts.
If we cannot find your private key which is stored on your mobile device, we have no choice but to reset your wallet in order to create a new one for you.
This means you will lose any previous tokens you have earned, but if you lose your private key there is nothing we can do to restore it.
We plan to provide a way for you to backup your wallet keys in an update coming soon.