I have a Delta Electronics DPS250QB-4A Power Supply. The computer wouldn't come on, so I removed the power supply completely. The green power light would not come on. Then later, when I plugged it in, the green light would come on. Then later (all still unplugged), when I plugged it into the wall, now the green light doesn't come on. Is the power supply bad?
First of all I have no idea what you mean by green light....
On the power-supply near the cable where it shows if it receives power? On the Motherboard?
Easiest way to know is if you're able to get a spare powersupply from another computer real quick either a friends or another one you have 5-10(More if you've never opened a computer) minutes of work unhooking cables and you'll know for sure.
If that's not an option there is probably a way to check voltage with a multimeter but that's not something I can assist with.
I'm guessing nothing happens when you hit the power button? Usually if the power supply was fine then at least the motherboard would beep unless it's totally dead too.
If nothing happens when you hit the power button, checking the jumpers to the motherboard from the front power button may be good. How foolish would you feel replacing a power supply then finding out that's all it was, I say that because I've done it.