Black screen could be caused by a number of reasons. If it is an old PC and isn't worth the trouble of fixing and if your main goal is just to retrieve some files then you can always take out its HD and plug it to another PC so you can retrieve your files. But if the HD is still in good condition then it can always be a secondary HD to your perfectly working PC.
Same thing happened to me and a friend recently: 1) My prob - hard drive failed 2) Other prob - monitor died #2 very easy to check for, just plug another monitor in and see. #1 requires some digging around google to confirm that's what it is.
If your hard drive is 4 years or older that's probably the problem. I went on vacation for 2 weeks, computer was working fine, came back boot up and get System 32 corrupted won't boot to windows. Tried everything, including a fresh install. Turned out bad HD, once replaced working great.