I believe that your tree may be gone. It looks young so if you can, just start another. The leaves look dried up. That's the cause of it being hit with too strong of sun for being that young or it could be that it's dead. Maybe try taking off some of the dead leaves, mist the living ones, and put some plastic around it like a clear bag for a week and see if there is improvement.

Your tree does looks dead. It should be treated like a tree because it is not a bonsai tree. Lightly scratching the trunk and seeing if it is green will tell you if there is any life in it.

I received a bonsai tree as a gift that I started growing from a seed. He was doing great and then I left him with a friend whom I believe overwatered him and he got slightly brown. I let his soil dry a little before I put him back on his regular watering. He hasn't grown since and while some of his leaves are still green they are falling and crispy.

Is there any hope in saving him? I'll attach a picture.

He's a Japanese Pine