This is the issueL
"BootMGR is missing"
The BIOS does detect your HDD, so it is not missing or defective.
Something happened to alter the system.
Just how did you change the CPU?
Do you have a 'Live' CD that can loot athlete system for you?
You did not say what OS you are using.
Suppose it is Windows 7. You should have made a Windows 7 recovery disc when you first got the machine. Have you tried it?
Or, if you have a Windows 7 install disc, boot it up and look for:
“Repair your computer” in a menu item. Read the instructions.
Does this help any? Questions?