under the terms of the sale of goods act, and in this case you should contact them to discuss the way forward. If the monitor is beyond repair you'll be entitled to a replacement, but of course the retailer may not have that model in stock - in which case it must come from the manufacturer.
In practice, most retailers refer you directly to the manufacturers in the case of monitor problems, because they (retailers) don't have any way of making repairs. The monitor will be covered by the manufacturers' warranty anyway, and in many cases they'll deal directly with you. Your first recourse is to the retailer however, and the sooner you make that approach the better.