I would take the server to a minimum to POST configuration and check for behavioral changes, then proceed to add one component in at a time until a failure is identified.   Minimum to POST for the R740:   You may also find this article helpful for troubleshooting no power/no post situations: [URL]
I got a server r740, but it fails to start. PSU's are green but at the front panel of the server the power indicator is yellow and the server is not powering on. I need some tips around this because i already reset the PSU's and the problem persist.  
Server r740- Power Fail