Hello there, I am the Expert that will be assisting you today. My name is Norman-Wynne Damstra.
Welcome to Fixya, Debdeb_64.
The power supply could be ok, this little green light that you see on the rear of the tower case is likely to be the Ethernet/Network connection.
Possible reasons for failure are as follows:
It could be a loose connection at the 24Pin or Processor 4Pin power connector.
A damaged power supply, is possible.
A faulty power on switch.
A dead mainboard.
A loose connection.
I have published a few guides that should help you in this regard, they are entitled:How to Remove the Side Panels of My Computer Chassis!!!How do I reset my BIOS/CMOS to defaults?
and most importantly My Computer is not turning on!!! Is it dead!!! No POST!!!
Do contact a computer technician to test components if need be.