I may have a fi for you buddy.
I have fied like 10 computers with that same problem.
The question to your answer....Can your PC sitting without being on for a long period of time effect this..... YUP!!!!
WHY? when your computer just sites still and nothing is going on, heat and a mix of cold can cause your motherboard, harddrive, pretty must devices in a computer to get corrupted drives.
When you told me about the lights I't click.
Look in your PC by a silver battery..
Push the little switch on the battery foreward so the battery can pop out.!!
THIS IS THE FRUSTRATING and HARD PART.
If you look somewhere by your CMOS Battery.. you will see 2 tiny pins, then 3 tiny pins.
depending on your system it may be 2 pins then about 6 pins
The side with 3 pins take the little plastic covering of the pins (Jumper) move it to thheeeee ummm Right... so that the Jumper is still one of the original pins it was on, but also is on the other.
Let this sit for about 10 minutes. then move the jumper back to normal then put the CMOS Battery back on it...
This should mpst definitly work.
EXPLANATION::
When your computer first boots it boots into what is called POST (Power on Self Test) its the back screen that shows what is booting when you turn your computer on.
The CMOST Battery contains all that info to boot.
What may have happened during the time your computer was sitting, your CMOST may have gotten corrupted causing your conputer to just sit like a stife doing nothing.
It wont even turn on on most NOTES. Not even a green light
Gosh this is long!!