Recently had a important machine quit booting on us and we suspect it was from a bad windows update. Not 100% sure but its a good guess. After searching we found a great article on Toms Hardware site. Below is part of the article.
Fixing the Master Boot Record (MBR)
Step one: Turn your computer on, booting from either your Windows 7 Installation DVD or Windows 7 System Recovery Disc. Remember, you may need to change the boot order inside your BIOS to have the your DVD drive boot first.
Step two: After the installation or recovery disc loads, if prompted, select your language settings and then continue. If you are using the installation DVD, when prompted by the following screen select Repair your computer.
Step four: Once you have reached the System Recovery Options screen, as shown below, you will be faced with a list of choices that can aid you in repairing a damaged Windows 7 operating system. If you wish to try the Startup Repair option first, it is often successful in automatically fixing many different start up issues, but in this article we will be using the Command Prompt option to resolve our problems manually. So, click Command Prompt to continue.
Step five: Now sitting at the command prompt, enter the following command and then press enter:
If successful, you should be greeted with the message The operation completed successfully. That’s it! Your Master Boot Record has been repaired.
While the above command does fix the MBR, and sometimes that is enough, there still might be an error with the system partition’s boot sector and Boot Configuration Data (BCD). This might occur if you have tried to install another operating system alongside Windows 7, such as Windows XP. To write a new boot sector, try the following command:
If you are still faced with your Windows 7 installation not being detected during start up, or if you wish to include more than one operating system choice to your system’s boot list, you can try the following command to rebuild your BCD:
The above command will scan all your disks for other operating systems compatible with Windows 7 and allow you to add them to your system’s boot list. If this fails, you may need to backup the old BCD folder* and create a new one in its place with the following commands:
*Some users also find simply deleting the boot folder and retrying the above steps effective at resolving boot issues, but it is not recommended.
Source Tom’s Hardware