If a computer won't boot at all (blank screen) or gives some type of error during the process, there's a good chance it has something wrong with the MBR. A corrupted MBR could happen in a number of different ways including a hardware failure. If the disk has developed a bad block in the MBR portion of the disk, you might just have to get a new system disk. Wikipedia has a good entry explaining all the details of the MBR and what you should expect to find if you go looking.
There are essentially two ways of creating a dual boot system. Both involve installing the different operating systems into their own partition either on the same disk or on separate disks. The easiest way only works if you have two hard disks available. In this case you simply put one OS on the first drive and the second OS on the second drive. The installation process is then free to use all the space available on that drive.
The problems tend to start when you do the multiple installations. As a general rule the last OS installed typically loads its boot code into the MBR while potentially ignoring a previously installed OS. This is a common occurrence when installing Windows after Linux. SGD can easily fix this problem with a few simple menu commands. Windows antivirus software can also get in the way and remove your GRUB boot.
The SGD website has a number of different sections to help you solve your boot problem. A good place to start is the SGD wiki. Here you'll find a wealth of information and troubleshooting tips on what to do when your system won't boot. There's a section on Boot Problems and their Solutions covering all the common scenarios and what to do to fix it. One quick look at the site and you can tell the wiki, in its current state, is definitely a work in progress.