Pretty much the same problem here.When I did the 'powercfg -a' it said only S3 was available, and in my bios, there doesn't appear to be any options at all to modify sleep settings (aside from turning on/off S3).
In my case the sleep mode was working just fine couple weeks ago.
Also my restore points date back only about an week, so that won't help me.
Problem started when i installed new graphics card drivers from ati/amd.
For some reason the install had failed or something since my graphics card was suffering from error 36 after the update.
It took me quite a while to get rid of that error, but when i finally did, i noticed i was missing both hibernate & sleep mode options.
Some specs below.
AMD Phenom(tm) 9550 Quad-Core Processor (4 CPUs), ~2.2GHz
984 GB Total
AMD Radeon HD 6700 Series
Windows Vista™ Home Premium (6.0, Build 6002) Service Pack 2 (6002.vistasp2_gdr.110617-0336)
- Aspire M5201
- AMD Phenom(tm)9550 Quad-Core Processor
- 8192 MB
- Graphics Card(s)
- AMD HD Radeon 6700
- Monitor(s) Displays
- BenQ G2220W (Analog)
- Screen Resolution
- Logitech USB Mouse
- Acer Standard USB Keyboard
- Internet Speed
- 15 MB/S