You've probably encountered this one before, both from non-Christians and Christians. The biblical response to this is very simple and clear: People don't go to hell for what they don't know but for what they do know.

Paul said that "since the creation of the world His invisible attributes, His eternal power and divine nature, have been clearly seen, being understood through what has been made, so that they are without excuse" (Rom 1:20). Psalm 19 tells us that the whole universe is shouting with evidence that a holy and just God designed everything we see. People don't go to hell because they didn't hear about Jesus. They go to hell because they sinned against the God they know exists. 

But some will still respond with, "I just don't see how God would send people to hell who never heard about Jesus." If what the objector says is true then we need to stop witnessing, because after all, if people don't hear about Jesus they get to go to heaven. So why tell them the gospel and risk them rejecting it?