Castiel didn't remember the impact. It was sudden and jerked him and the other two people in the car to the right. Smacking his head against the window so harshly it shattered under the force of the impact.
He also didn't remember what happened after, though Sam did try to tell him that they all just sat in agony for a while, as the bus driver and his passengers called for help.
He didn't remember being put in the ambulance, but he did remember waking up halfway there and the paramedic demanding he stop moving, because he was lucky to have even been alive.
He didn't remember much of the first three weeks after the accident, they said concussions can do that, but He did remembered pain killers. Lots and lots of pain killers.
He didn't remember the times Sam came to check on him, just to make sure he was still breathing. But he did remember hearing a doctor talking to a nurse outside his door about how one of the boys in the crash didn't make it.
Looking back on it now, he couldn't remember how long he had been in the hospital, but he did remember having to leave in a suit so he could go to his friend's funeral.
As a side effect of his concussion, he couldn't remember that this person used to be a lot more than just his friend. But he did know that somehow, this empty feeling in his chest would never be filled.