Our server: HP Proliant ml350p g8 has a single Xeon e5-2609 1.4Ghz processor.
Our manager wanted to update it a little, so we looked at 2x E5-2609V2 processors...we ran it past HP Enterprise chat who said it was compatible.

We now have the new CPUs and when we installed them our Health LED was solid red and server wouldn't POST.
We've tried each of the new CPUs individually and together but same result.
We thought perhaps it was the BIOS (it was dated 2012!). 
Our server is out of warranty so we initially wasn't allowed to download the latest BIOS, but our manager persuaded HP to give us the latest BIOS version (2015), which we have installed.
Trying the CPUs again has the same result...server still won't POST

If anyone could please shed some light on what we have missed that would be great.

(We are in the process of getting quotes for HP Maintenance Contract - since we are past 30 days expired on our Warranty, so we can't get a care pack)