Was there the possibility of a power surge affecting all four PCs? Are they connected to the mains/telephone lines via surge suppressors? Have you had any storms in the area recently? Otherwise is there a possibility they've all been infected with a virus (which is programmed to trigger on a specific day)?
These are 2 reasons which come to mind why a bunch of PCs may all suddenly misbehave without warning on the same day/time. I'd also check that if they're all powered through a common power supply or are using anything in common (eg a KVM switch) then all of those connections are in order.
When you say no power LED, you must have power or else the fans wouldn't be turning. What happens if you unplug them from the mains for a few (30) minutes and then plug them in again and make another boot attempt (perhaps using a different mains power socket)? If this doesn't work what about unplugging them again, removing the motherboard coin cell for a while and then reinstalling that and plugging them back in - you'll need to reset the BIOS settings but it's something you can try in order to revive them.
all these systems were brand new. Just weeks older!! Not even used.,
Just powered on for routine check up ..
The power factors and Earthing were good enough!!!. one thing i have noted is a USB to PS2 converter connected to use the KEYBOARD AND MOUSE FROM SINGLE USB CORD, Not of good quality, Some times im getting error msg, like Exceeding the Power limit. Does it make the system to behave such a way?
all these systems were brand new. Just weeks older!! Not even used.
Then why are you posting here - contact your supplier and do it right away! Never mind you trying to figure out error messages, 4 DOA systems warrant immediate attention from the place that sold it to you, that is if they care at all about their reputation.