how is DIO alive after he was stuck in Jonathons arms and how did he get into the secret compartment without erina knowing

Right after he realized Jonathan was dead, Dio tore off his head with his hair/neck tendrils and proceeded to put his own head onto Jonathan's body. He then dragged himself into a secret compartment inside the same bomb-proof coffin that Erina used to escape.

After Erina was rescued, the coffin was cast back into the sea, with Dio still inside. He presumably went into a state of hibernation. He waited there for nearly 100 years, until a group of fisherman off the coast of Africa happened to reel the casket in. Thinking they had discovered treasure, they opened the casket and awoke the vampire, who then christened himself as simply DIO.