Okay, if everything is connected right, cmos, hard drive jumpers set correctly, (Master I'm assuming) Hard drive is connected to IDE 1. There are some other possibilities why it won't boot. If you can get into your bios settings, Make sure the motherboard is recognizing the drive. make sure that your drive is in the program's boot sequence. You may have to tell the BIOS to search for hard drives (it should do so automatically).
Here's another possibility: Your power supply may be inadequate for the new motherboard. Check your motherboard's documentation to find the wattage the vendor recommends.
If the power supply checks out, boot your machine from a floppy to see whether all the necessary files are accessible on your hard drive. If it uses the FAT32 file format, any bootable floppy disk will do. If it uses the NTFS format, download (on another system) NTFS.com's free NTFS Boot Disk utility , which will prepare a bootable floppy that can read NTFS hard drives.
If you can access the data on your hard drive but you still can't boot from it, you may have to suck it up and reinstall the bastard.
I hope this helps and keep us updated.