3 years ago I delegate XTZ to a baker using TEZBOX. Today I found out that the TEZBOX wallet is not available. I migrated to Temple wallet using a Privat Key, but now I have 0 XTZ on my balance, all my money is on a smart contract KT1, which has been not active since Sep 2021. When I try to Import an account using Managed KT, I get an error: You are not a contract manager. Please help
I have 0 XTZ on my balance
This is either due to the way you imported your private key (e.g. you imported a bad tz address, but this rather happens when using a seed, not when using a private key).
but now I have 0 XTZ on my balance
Assuming you correctly imported your account/private key, the fact that you have 0 XTZ is not related to Tezbox, but rather to the fact that you likely emptied your account.
all my money is on a smart contract KT1
You should get a few XTZ (less than 1XTZ), to interact with the KT1 and transfer back you assets to a tz1 (knowing that tz addresses are now (since a long time actually) delegatable).