I agree with rharvey pretty much, and have a practical suggestion.
When you have a system boot related problem I suggest that you always do a few things right off the cuff.
Pull the HD and slave it to another known good working system.
View the HD in BIOS to ensure it's visible and doesn't have a corrupt MBR/Boot Sector, etc.
If the HD is visible in BIOS, run a HD Diagnostic tool on it to determine it's general health, and if it's found to be corrupt or damaged immediately power it off to prevent further damage to the data and get your HD Recovery Solution/Provider involved if necessary.
Never continue down a path without a process to the madness. In the instance of "solving" HD related issues you are putting your clients data at risk with ongoing use of the drive, if it's failing physically, or if you're using an untested/vetted process to fix the real issue.
This places your company and/or you, into an unwanted liability scenario for that data. If your clients are not already signing a service waiver that includes a clause to protect you from the unintended loss of their data, you should.
Your first step, with every computer should always be diagnosing the health of the HD in my opinion. Having a machine come into your shop, or providing service remotely, to a machine with unknown issues always implies the possibility of these risks. If you're not actively monitoring, and managing the system, can you within reason say to yourself that the system couldn't possibly have at least a 1% risk of having this happen, and if it does happen, are you not going to be the one held accountable?