If you find out that there is nothing on display and there is no response from the computer, the first and the foremost thing to do is get the source of the power checked. The problem may have been caused by a loose connection. The wires should be checked as well. If you have a separate monitor, you have to replace it with that one and turn on the system.
If you see that the monitor is working fine, you will know that the problem is somewhere else. In most cases, it is case of RAM failure. There is hardly any way out but a replacement in such a case. However, the problem may be accused by a mild dislocation and so adjusting the position of the RAM may be effective in this case. The fans of the motherboard should be checked and it has to be made sure that the CMOS battery is properly seated.