Neither is either correct or incorrect. They are both correct, in context.
'Can somebody help me?' = Out of all the people who hear this, I need help from somebody.
'Can somebody help me?' = I've already asked once. There must be somebody who can help; are you really going to ignore my need?
'Can anybody help me?' = I really need help, and I'm not choosy about who offers it/
You can mix these together: 'Can somebody help?... Somebody?...Anybody?' (The "..." are pauses - not just me not being bothered to fill in the rest of the question! )