I have installed Windows 8 on my Mac. Now, when starting up and selecting Mac, it begins to start, then shuts down
I did something similar and ended up incidentally tweaking the partition table from inside of windows. Because of that it wrecked the EFI and GUID and nearly bricked me completely I thought. Start the computer and hit command-r and you should be able to tell where to go from there. I have to warn you though its probably going to involve wiping one or both partitions. If the EFI table has been altered you're going to have to wipe and clean install to get it back to normal working conditions. Hopefully you can just verify and repair your disk though and that'll do the trick.