After installing the processor and reassembling the computer, the system begins the boot process and suddenly turns off before completing the boot. what should you do?

Explanation:

Information is not complete for diagnostics, but enough to suggest next steps to get more information.

Assuming modern Windows type operating system.

Assuming "begins to boot" means that computer passes all POST diagnostic tests, then system has found the bootable drive according to boot order, and "begins to boot".

Since all POST tests passed, the CPU is correctly placed and functioning correctly.  The reassembly should be reasonably fine as well, since the booting medium also starts to work.

Some possible hardware problems could be:

1. Open the case and check that all components are in place and properly connected, especially the CPU-cooling system.  Reseat all connections.  Remove excessive dust, if any.

2. Memory chips not seated properly.  Reseat memory and try again.  

3. Check for proper CMOS settings which may have been lost during the installation and reassembly.  If not, enter correct values, or reset to factory settings and start from there.

4. CPU overheating during the time of POST diagnostics if the cooling fan or cooling fins are not reinstalled properly.  Generally overheating problems will cause system to reboot, which is not mentioned, but not ruled out either.  Check for adequate cooling configuration (cooling capacity for the new CPU) AND for proper installation of the cooling system.

5. Try to boot with a bootable USB drive, or CD.

6. If the system cannot restart (POST test, etc), then some components might have been cooked, could even be the power supply.  Remove peripherals not required for booting, and reseat remaining components.

7. If previous steps don't resolve, or no change, replace removable principal components one by one to check for defective item(s).

8. If none works, by elimination it could be time to replace the mother-board.

If user had given more specific information, steps may be shorter.

Basically the same hardware trouble-shooting rules apply for Linux systems.  The software rules might have different names when booting is successful.