Righty, click on the link below for the information on the M2N68-LA Motherboard.
http://support.hp.com/gb-en/document/c01925534
Check in the cpu support section on the web page if the new Cpu you put in the motherboard is listed and supported
If it is you, will see what bios revision of firmware the board needs to be to fully support the new cpu.
If your version of the Bios firmware of your motherboard is not the correct version number to be able to support the new cpu.
You will first have to put your old cpu back in.
Download the bios firmware update. flash your motherboard to the correct revision to support the new cpu.
Then simply put the new cpu in the cpu socket of the board and it should work after that process.
M2N68-LA
Processor upgrade information
Motherboard supports the following processor upgrades:
Socket type: AM3
TDP: 95 watt
AMD Phenom II X4 9xx/9xxe/8xx Quad-Core (Deneb)
AMD Phenom II X3 7xx/7xxe Triple-Core (Heka)
AMD Phenom II X2 5xx (Callisto)
AMD II Athlon X4 6xx/6xxe (Propus)
AMD II Athlon X3 4xx/4xxe (Rana)
AMD II Athlon X2 2xx/2xxe (Regor)
AMD Sempron 1xx (Sargas)