While orange juice is a simple sugar, and appropriate for hypoglycemia, it should never be administered by someone to an unconscious or semi-conscious person. The orange juice should be handed to the person for the hypoglydemic patient to self-administer, and if they cannot do so then do not use orange juice. An alternative is glucose paste administered to the mucosa inside the mouth. It comes in a toothpaste tube. Otherwise call 911. The reason is that in an unconscious or semi-conscious patient a little fluid aspirated into the trachea can cause a spasm, completely blocking the airway, If this happens, and the caregiver is not trained in airway management they may die. Most drowning victims die from this and have no fluid in the lung.
By the way - waverider is likely 100 percent correct in his analysis, but sometimes you have more than one thing going on - and in the event of a stroke, only a limited time to take action before permanant damage is done.
First of all, someone should have access to his blood sugar meter and know how to take a test. This sounds like a situation where 911 should have been called. I don't mean to monday morning quarterback, but grandpa needed an MD to evaluate his condition. There is no possible way to determine on th internet if the problem was due to a stroke of a blood sugar issue. He needs a thorough evaluation, to include a neurological work-up as of yeaterday. Loss of consciousness of unknown etiology mandates a 24 hour observation in all hospitals I know of in the United States. Generally a diabetic coma has a gradual onset. The problem could be cardiogenic, neurologic, and possibly due to the diabetes. I would get him to an ER immediately.
"What happened to him?"
I'm no doctor but it sounds like your grandpa had an hypoglycemic [low blood sugar] episode. Your parents did the right thing at that moment by giving him something sugary - orange juice - to raise his blood sugar levels. If this was not the first time he fainted, or lost consciousness, he needs to be evaluated by his doctor for medication adjustment/s, and/or to look for other hidden health issues.