it would take quite a rough monitor change to dislodge a ribbon cable seeing as many i have come across where people have ruined the connectors from trying to pull them out as they are real tight.i would of thought if the pc is booting up to displaying invalid system disc then it would suggest it finds the hard drive but no boot files on it,which would suggest corruption or hard disk fault.hopefully the op will let us know what it was when the problem is solved.
simply me
if it will not start in safe mode then you will have to boot the pc from the cd rom drive.first you will have to set the cd rom drive as the first boot device in the systems bios,if you are unfamiliar with installing windows and setting the bios etc,then i suggest you get someone in the know or a shop to install it for you.
simply me