Same thing happened her i had a year and a half old boxer that jumped out of the back of my truck and started chasing javelinas and i called him for about an hour then i figured that he might have ran back to my house but when i got there he wasn't there so i went back to were he was and i found him laying on the rode so i got him to the nearest creek and cooled him off then i took him back to my house and he seemed to be coming back around and then i slightly moved him and he went into seizers we got half way to the vet and he quiet breathing we got to the vet to see if there was any hope but the vet said he was to far gone so i know how it feels it sucks it took me about 4weeks to get over mine and sometimes i steel get memories of him and i teer up. They vet said it is real common for boxers to die at an early age because they have heart attacks.