What could be the reason of sudden fainting?

hello
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He could have fainted because of one of the following causes

1. He had a hypoglycemic attack ( in case of starvation)
2. He had a syncopal attack

If you think he hit his head against something then look out for the following signs

1. severe unbearable head ache
2. vomiting
3. further loss of consciouness
4. seizures
5. weaknees or loss of movement of any of the limbs

in case any of the above symptoms consult a doctor immediately

Hello Dear...
Thanks for your question.
First of all I must say .... Do not panic in such situation... Almost every person has fainted sometimes in his life . Did he have similar episodes before ? Did he have tongue biting or incontinence during the episode? There is every chance that he was having an attack of epilepsy. There are multiple causes of sudden faint ranging from dehydration , hypotension, hypoglycemia, to more serious causes like epilepsy , Cerebrovascular attack etc. A proper neurological evaluation is needed. You must take him to a neurologist and get a checkup done.

I hope everything goes well & fine . Update here about how things progress..
Take care.