The protocal itself is extremely secure. What the blockchain says goes as far as wallet ballance for every single wallet on the ledge are concerned.

As far as your password or wallet.dat file that is your key to your specific wallet, that is as secure as anyones passwords are. Your funds are as secure as your password. The major difference between blockchain digital currencies and traditional currencies is that you dont need to have your password stored somewhere digitally. With banks it is stored on their side of the database or on your computer which is subject to hacks and data breaches. With blockchain you can store your password on a piece of paper off the computer as a "cold wallet" and access the wallet only when you need to. So unless they steal your piece of paper that holds the key they wont get in because there is no breaching the blockchain protocols side that validates your password.

Of course this is less convenient, most often convenience is the cost of more security.