When you say that it "doesn't come on", I presume that you mean that nothing lights up on the control panel, so, depending upon the make and model of the unit, there may be a fuse mounted on the mother board that has popped and needs replacing. Replacement will require removal of the control panel, and the fuses are not just simple automotive style glass fuses , but rather proprietary types, so you need to open up the panel, find the fuse, test it for continuity, and if it is blown, find a replacement. The least expensive place that I have found for replacement dishwasher thermal fuses is ebay. And make sure that the fuse you purchase is NEW, and not just a used one salvaged from a scrapped dishwasher