Unless you are a very experienced violinist, fixing or changing a violin string is a challenge. Violinists know how to tune each of the violin's four strings, so changing and tightening a string is easier for them. Less experienced players or non-players are advised to learn the parts of the violin, the string tunings and how strings should be wound before attempting a fix.
Checking the String, Tailpiece and Peg
Check to see if the loose violin string is broken. If so, carefully remove it from both the peg and the tailpiece. Get a new string of the same type.
Look at the violin's tailpiece, which has either holes or metal holders for the strings. Make sure the string you have will fit the holders on the tailpiece. Try putting the knotted or metal end of the string through its tailpiece hole and see that it fits. If not, you may have to get another string that does fit.
Check that the peg for the string you are fixing moves easily but not loosely through its holes in the pegbox. If it is very tight, use violin peg compound to make it work smoothly.
Fitting the String on the Violin
Find the hole drilled through the peg closest to the middle of the peg. Make sure the top of the string, usually wound with thread, will fit in the hole. If the hole is too small, it may need widening or replacement at a violin shop. If the hole is too big, the string will not stay wound, so the peg should be replaced.
Take the lower part of the string and fit it in the hole or holder on the tailpiece. Hang on to the string to prevent it falling out of the tailpiece while you push the top end of the string through the hole in its peg, until about 1/4 inch sticks out of the other side of the hole. You may need tweezers to pull it through.
Wind the string around the part sticking out until it folds under the rest of the string. Hold the string up so it stays, winding the string slowly toward the peg handle side of the pegbox until the string is almost at its tuning note. Make sure the string is fitted on the bridge holding the violin strings up from the instrument. Tune the string and make sure the peg is tight enough but not too stiff to move.
When you change a string, it is a good time to clean the area under the string and check that the areas the string touches are in good shape. Make sure you have spare strings available.
Avoid changing more than one string at a time. First, if more than one is changed, you are likely to spend much time tuning while it breaks in. Second, if you remove too many strings, or all of them at once, the bridge may fall, and the soundpost inside the violin may fall or move. Either of these consequences can cause damage to the instrument or cause it to sound bad.