A strange thing for me is I never had a problem with sleep mode. Well, actually, the only time I had a problem with it was actually a very obscure issue with the RAM I bought. If you want to read the details, they'll be in the spoiler below.
Aside from that, if sleep worked fine and now it doesn't, I would suspect a hardware problem over a software problem. Unless you've done something to the system recently.
So about that RAM. I built a system that had trouble waking up from sleep. The symptom was basically it'd go into a restart cycle. Gut instinct was PSU, as PSUs gave me more issues than not. Replaced it but nope. Second guess was to replace the motherboard... but the issue persisted. I couldn't say the CPU was at fault, because of my firm belief that CPUs are shipped with such low failure rates that I could probably have a shot at the Mega Millions Lotto.
Why I didn't suspect RAM at first? It kept passing Memtest. But I did anyway and low and behold, it was the RAM.
Now don't count me on this one, but MPC said in the past that RAM is usually the first thing to screw up, if something screws up.