2. If there is no diskette or CD, I would highly suggest you verify that all your cables are properly connected since you replaced your motherboard.  You may just have a poor connection to the hard drive.

3. If there is power to the drive (feel the outer casing (NOT THE CIRCUITRY) of the hard drive case to feel if it is spinning up on the inside). Then check to see if your computer can "see" the hard drive you have connected.  Go into the Computers Motherboard setup (This is the BIOS, and you often can enter it by hitting the DEL, F1 or ESC keys - I would need to know the exact model of your computer's motherboard to be able to give you the exact key)  Once you are in the BIOS, check to see if the motherboard can see the make/model of the hard drive that is connected.  If it cannot, then you may have a bad hard drive cable, a bad hard drive, failed motherboard, etc.  If it can, move to the next step.

4. Then it is likely that your hard disk's partition has been wiped.  A partition (table) is used by the computer to determine what is stored on the hard disk.  In this case, the old partition told the computer that it contained a bootable operating system and that it could be used to bootup the computer into Windows.  With it gone, you will have to get the partition recreated or fixed.  There are many tools available to fix partition tables, but they vary in their success.  (if the data on the drive is critical, I suggest seeking professional help to get your data backed up to another computer if you have not already done so already)  If you are not able to fix the partition, or if the data on the hard drive is corrupted/gone, you will have to reload Windows or another operating system to get the computer back to a useable state.