The more information you give us, the better suggestions we can provide.

Laptop or desktop? Make and model number? Operating system installed? What type and wattage of PSU did you buy? (Single rail units are the easiest to configure because they usually supply the correct voltage where it needs to be.) Since there are lots of versions of "will not turn back on", EXACTLY what is occurring? Do the fans turn on when you press the "start" button or does nothing happen at all? Is it plugged into a UPS backup battery supply? (Tried plugging straight into the wall?)

Hope this helps and let us know more.

Grif