Loss of computer use

Having recently bought a new Samsung computer with Windows 8 installed I took it to PC World and had W8 removed and W7 installed and everything was working fine.I then contacted Samsung to ask why "Recovery Solution 5" was not working and was this due to having removed W8.Included in the reply from Samsung was a paragraph advising me to press F4 while booting and I would then find out if this system was working in W7.I complied with this advice and now the computer has ceased to work.Spent best part of an hour with one of the Samsung advisors but to no avail.He said he would make arrangements for my computer to be collected but implied I would have to bear the cost of repairs.Can anyone help?.

The model is a Samsung NP350E7C-SO2UK,S/N HYCR98FD1C514EY.When turned on there is only a blank screen showing.The hard drive indicator light on the front of the computer lights up when the power button is pressed then goes out.The only thing that is active is I can access Bios by pressing F2 when pressing power button on,nothing else works.

My reply to Mark Phelps is that what you say is correct,but please remember this mishap is a direct result of advice from a source where you expect correct advice,is that not so?.

Was talked through all what you suggest by the Samsung rep. but nothing worked. The disk would whir away but nothing would appear on the screen.

Have done that but the trouble is it refuses to stay put.