My five year old girl has no appetite. How do u get her to eat again? Is there any over-the-counter vitamin that can be recommend.

Discuss with PCP. If your kid is growing at a normal rate & participating in peer activities, i'd leave her appetite alone. At 5 they are growing at a steady 3-4 lb/yr & 2-3 inches. They rarely eat a balanced meal but do have a balanced week.They often feed well only 4-5 days a month. If you want to buy vitamins, buy them, but recognize that you're likely just creating expensive urine.
Possibly. Vitaminb1 deficiency causes a disease called berberi and appetite was used to indicate recovery. In today's world, sugar is causing vit b1 def . I would suggest a supplement of b1, magnesium and multivitamin.