Determine the nature of the problem. Download Windows should start in a few seconds after turning on the computer. If not, pay attention to the screen. Sometimes the problem may lie not in the system, and the BIOS (the firmware that is installed on the motherboard) that is configured in the wrong way. If after switching on, the screen remains dark and nothing happens, the problem is likely just that.
Read the user guide to the motherboard. Find out which key is responsible for entering the BIOS setup, and press it immediately after turning on the computer. In the appeared menu, select Load Optimized Defaults, press F10 and press Y. So, you will restore the initial settings which could be changed at random.
Let's see if the system boot (you should see a loading bar). If not, restart the computer and repeatedly press F8. A menu will appear boot. Try to run it, selecting the option "Safe mode". If the system is loaded, the main menu, select "data recovery systems" and do a roll back Windows to a restore points. Restart the computer in normal mode.
Use a bootable Windows disk which was used to install the system on your computer. Insert it in the drive. In the BIOS, select option to boot from CD or DVD. Restart the computer and follow the instructions that will appear on the screen. Eventually you will go into the recovery menu system. If a rollback does not help, then the only solution would be to reinstall Windows from the boot disk. Remember that if you make it with the option "Update", then saved on the hard disk data will not be affected.