So, from F12 you get a boot menu, yes.
In there choose the boot to DVD with a disk in the drive - what happens? Does it spin up and try the boot?
And can you confirm it is a DVD drive that it has - some 755s have CD drives which have caught me out!
If the internal DVD drive is bad and not reading properly then it will show the symptoms you say - no boot from a DVD. In this case try an external USB DVD drive - connected before you reboot the PC. You will then be able to boot from it on the F12 menu.
