This started about 3 weeks ago. The screen would go black if I let the computer idle but it wouldn't wake back up. I could hear the fan running and the white power light was burning steady. I had to physically power it down then back on. I'd get the screen asking to start with command prompt, etc or start normally. A couple of times it asked to do a repair. The chkdsk ran twice. I had to do a reformat and reinstall but that hasn't repaired the sleep/no wake up problem.
Several times, when I would try to access the computer after it had been sitting a while, the fan would be running continuously. Is my hard drive going bad? My power plan is "balanced."
Use one of the linux discs provided by some magazines, operate as live and give the hard disc a complete cleanup and then a machine specific reinstall of its own operating system, by my own experience worked. Hope that helps Alternativelly you could download linux and do USB boot Or you could also simply turn the sleep mode off and that way your problem will be solved for good without hassle of resintalling everything.