This experience could be caused by a few different things. One of which could be losing consciousness and then coming to. Another explanation could be that Marcus got a concussion when he hit his head. A concussion can cause headache, blurred vision and seeing spots. So, it's possible his vision was very blurred and he was seeing the sun or some other light that made it look like he was looking down a tunnel. It seems more than likely that the light at the end of the tunnel was probably caused by him losing consciousness, though.