Without knowing private public key any one can withdraw btc from address. Is it possible?' Actually I am using blockcypher API for creating address of btc and storing all records in my database with private and public key of individual address and assign that to particular users.
But few days before from my users address balance will become zero.Amount transfer into some other address.How is this possible?
Please help me out from this Its very risky.

Thanks in advance.
Regarding your title, without knowing the private key, no one can withdraw your BTC, that's for sure.

First of all, I would argue it's a bad idea to use a third-party online service to generate and manage your keys. I went to check the blockcypher site, it seemed legit. Did you spend your coins? If so, the money may be moved to a new address because the change address is different. They've said in the docs,


  BlockCypher will set the change address to the first transaction input/address listed in the transaction. To redirect this default behavior, you can set an optional change_address field within the TX request object.