Sound is produced by the closure of the vocal fords (cords) when air passes through. This applies to both singing and speech. This is PHONATION.
So why this definition? Most singers don’t realised that the vocal fords (cords) are part of the human body and for that matter when their body is tired, they wouldn’t be able to sing.
If you loose your voice frequently, then, it could be one of these:
*You’re sick knowingly/unknowingly
*You aren’t eating the right food.
If you you fall into the first, take days off work and REST. If the second, see your doctor and if the third, see a Voice Anatomist.