Some bitcoin got sent to an old deposit address, can i get this bitcoin back or is it lost?
It depends what your wallet was stored on. Blockchain.info, CoinBase, Bitfinex, BitcoinCore. They all have ways to recover your "private keys". The only way to redeem your Bitcoin if you've lost your wallet somehow (and the wallet isn't a browser wallet) or etc, is through your private keys.
Here's some info on how to retrieve your old Bitcoin wallet
One person asked a question before you about this as well, check it out.