Although vehicle was turned on and in drive, it would not move forward.

What is your question?

Last Friday I turned my Saburban on and put it into drive. Everything was fine as I drove a short distance, until came to a complete stop at a red light. As the light changed to green, I released the brake to creep forward to allow on coming vehicles to pass before attempting my left turn. As the last on coming car passed me by, I steppe on the accelerator, but the vehicle would not move forward. I could hear some grinding, then suddenly, it felt as if I had been hit hard from behind and my Saburban jerked forward and was now able drive. I pulled over to exchange documents with the person who had hit me, but to my surprise, no one had hit me and there was no damage to my vehicle, nor was there any car behind me as I felt the strong forward jerk. So, the problem lies with the vehicle. 1.) Why would it not engage as I stepped on the gas pedal? 2.) Why did I hear a loud grinding sound? 3.) What may have caused the strong push forward? If you have any ideas as to what the issue(s) may be, please advise?