SanDeGo has exceeded 63,600 blocks, and changes in rewards are applied. So, wallets which are version 2 or lower than that cannot connetc to the correct chain.
You have to necessarily update a wallet to version 3. If you have not done yet please download and use it as soon as possible.
https://github.com/sandegodev/SanDeGo/releases/tag/3.0.0