One idea is that the type of partitioning was changed. I am not sure how just shrinking the OS X partition alone would cause Windows not to boot. There are also third party tools, such as Winclone, that can be used to backup a Windows partition, shrink the OS X partition, create a larger empty Windows partition, then restore Windows to this larger partition.
You can, for example, remove Windows completely and then reinstall Windows to a larger partition. Or, in other words, there usually is not way using OS X to enlarge the Windows partition after shrinking the OS X partition.
Usually, in order to enlarge your Windows partition, you have to relocate the entire partition.
