If you've already cleaned it well and the heating element still isn't heating up, try doing a test before sending it in for repair: take out the tray and put the toaster in a vertical position against the light to see if the upper heating element glows when you turn it on at maximum temperature (make sure it's unplugged first, obviously). If it doesn't turn red or orange, it's dead and needs to be replaced, but if it does glow then the problem could be the thermostat that controls where the heat gets distributed, and that also requires a technician. With six months of warranty you're probably still covered, so call Electrolux's service department and tell them you're burning on the bottom and not toasting on top, because that sounds more like a factory defect than normal wear and tear.