AMD processor model unknown... 99% of the time happens when a BIOS doesn't recognise the processor that is in the CPU socket. You may need to update your BIOS. Things to check :
1. What your motherboard is by either opening the computer up, or using some software. For example using CPU-Z
http://www.cpuid.com/softwares/cpu-z.html. CPU-Z will tell you what the manufacturer and model is under the mainboard tab.
2 Using the same application or looking at bootup (BIOS numbers along the bottom) look for the BIOS version, check on manufacturers' website that the BIOS version supports your CPU.
3 If not, then download and follow their instructions on updating to the latest BIOS that will support your new processor.
Hope this helps!