My 4 years old Compaq Presario c700 (C794) laptop is not booting up. Does not go into BIOS, and does not even show the message “Press F10 to enter BIOS” etc.

• I see a clear black screen (but has backlit and I am able to adjust the brightness using Fn keys). Tried on the desktop monitor, it is black too.

• The HD is running intermittently, all LEDs are on, and the fan is running.

• Windows is not booting up, there is no log on sound. WIFI light is also orange, it becomes green after booting is complete. HD activity is on but it is not as much to boot up Windows 7.

Troubleshooting I did till now:

• Hard reset by holding down the power button several times.
• Cleaned RAM and placed it in another slot.
• Removed the power and battery and kept it overnight.
• Tried on the desktop monitor, it is black too.

This happened all of a sudden. I did not do any BIOS updates or anything that normally causes these kind of problem. I learned from some forums that an exhausted CMOS battery can cause it. Is it true? In this case, can you please give the location of the CMOS battery in these laptops?

What are the other measures I can take?