There are plenty of Optiplex 790 with both i7-2600 and i7-2600k on Userbenchmark.  You may want to do a reset if haven't done so.

@Chuck93256 

2600 or 2600K should not be an issue as long as you.....

Updated BIOS to current BEFORE changing CPU

And you have a TOWER , Desktop, SFF model. USFF isnt going to support 65W to 95W cpu.

Last resort would be to get a unit with that already installed.

Dell-OptiPlex-790-SFF-Desktop/dp/B01HTTGI64

Try disconnecting all storage and turn on to post HDD not found error. The cpu may be fine but windows is not. Black Screen of death is very common now.

I do not think you need to update bios to run either i7-2600 or i7-2600k.  i7-2600 and i7-2600k have the same cpuid as far as bios is concerned.  CPUID signature: 206A7.

i7-2600 is supported on 790 from the get go when the model was released.

Can you put your old cpu back to see if the PC can still POST?

many possibilities why motherboard cannot post or apparently not able to.  First thing to double check is video cable connection and monitor function.

I forgot to mention the motherboard is a used replacement from a doner 790 

Also when I tried last time my windows 10 was not activated so I put back the factory CPU and activated it but haven't tried sense. Could that be why?

No.  When changing CPU's or any other hardware, Win10 will "detect new hardware" and ask for restart.  May be the i7-2600k is defective.

I am gong to try another chip. Thanks.