Originally Posted by GuitarBizarre

You need to buy some peg dope. Its available from anywhere that sells your normal violin accessories. You remove the tuning peg from the headstock and add a little around the peg hole. It provides extra friction and that will let you tune it again.

Common problem. If peg dope doesn't work you'll need a new peg, but thats not likely on a violin that hasn't been used much.