I am having the same problem with my HP 2511x as many others (as I have now researched). The monitor does not detect a signal to the pc. I have switch the DVI cables suspecting there was an issue with the cable, but had the same results. I then switched the DVI cables and replaced with HDMI cables and again ran into the same issue. I then bluggeda different monitor with DVI connection to my PC and immeidately had an image. Therefore the issues exists with the monitor and not the PC or the graphics card within the PC. I purchased this monitor about 10 months ago and certainly did not expect to get a LEMON from HP.
What I do not undertand for the life of me is why HP has not issued a recall for this model as there are a MULTITUDE of troublsehooting requests out there which all reflect the SAME problem. I suspect HP is quite aware of an issue with this model and I truly hope that they will either repair or replace at no additional charge to me.