monitor displays "please check signal cable"

About 2 weeks ago my computer was shut off improperly, it was shutdown by pushing the power button instead of by windows. I did not change any settings before it was shut down i.e. no setting changes.(it worked fine before) My monitor now displays the please check signal cable message. I plugged a different monitor and cable to the tower and still got the same message. The monitor is not the problem. I reseated the video card twice did all the trouble shooting with that idea, still the same message. I have a vga to dvi adapter do they just go bad all of a sudden? I tried the local stores to find one with no luck. I found one on the internet but I don't want to buy it if that is not the problem. Any ideas? Also would this be covered by the warranty?