Before, windows used to appear after about 2 hours or so. Just right now I turned it on, and after about 20 minutes, windows came up.
If you're saying that the display only comes up after a period of time, and it does this with more than one computer, it sounds like a power issue. Two possibilities: the power supply in the monitor, or the inverter. The most common failure mode that I'm aware of in lcd monitors is the capacitors in the power supply. I have an LG 19" I took apart and most of the capacitors in its power supply are shot, bulging tops and/or leaking fluid. The capacitors LG used were not the highest quality, by temp rating. They likely failed due to poor ventilation and cooling in that section and lower temp rating caps.
If this is the problem, which you can determine by taking it apart and looking for bulging tops or leakage on the caps, and if you're good at soldering, they can be replaced.
Another failure point on pre-led units would be the cold-cathode tubes. I think sometimes at least this can be seen as a very dark image (try turning out the lights in a dark room.)
If the image is coming up eventually, this is an important clue. I would suspect the power supply, but I'm not an expert and prepared to be corrected by someone more knowledgeable.