I've had my Inspiron 545 for a few years now and it was working fine untill today when It wouldn't turn on. I turned the computer off at night and in the morning it just wouldn't power on. I usually leave my computer on for long periods of time (12 hours a day usually), but this never had any problems before.
There is a green light at the back which is part of the PSU and it's on (does that mean the PSU is working fine?). When I press the power button the light on the button blinks on and simulataneously the green light at the back blinks aswell. Also I hear a small noise as if the computer is trying to boot up.
When there is a hardware problem early in startup the BIOS usually beeps to let you know what failed (e.g. memory or video or ...) Some PCs have a light sequence they use instead. Can you check your manual in the troubleshooting section and see if they have a description.
re: "...I hear a small noise as if the computer is trying to boot up...." does this sound like a disk spinning up or fan noise of fans blowing? or something else like beeping ?
Does the monitor ever recognize the computer is booting? Does it come out of low power state? Can you see any of the BIOS splash messages? Can you get into BIOS?