There are many altcoins like bitcoin gold and ethereum classic that have suffered 51% attack before, you can read the links given above to understand what 51% attack is, but to just make it brief, it is an attack carried out by hackers to compromise the blockchain of a particular coin, they have their own miners that is powerful and make it centralized in a way to steal coins from people, in a 51% like ethereum classic attack, you can have ethereum classic on you wallet but all gone because of the attack.
Is the blockchain network really safe from this sort of attacks?
Did you mean the bitcoin blockchain? Bitcoin blockchian is safe from 51% attack. When the blockchain was not so strong like this before, no attack has occured on bitcoin blockchain before.