I continue to see a lot of posts about wake-from-sleep issues with various HTPCs, usually experiencing a blank screen when attempting to wake an HDMI-connected HTPC.
I have the problem on my Assassin HTPC, and unfortunately it just seems like a largely-unfixable issue with EDID, HDMI connectors, hardware handshakes, A/V receivers, video card drivers, etc. I've tried all the various hacks, settings, workarounds, and voodoo, and even a hardware approach with Gefen's HDMI Detective, but nothing worked as a permanent solution for my setup.
I did discover awhile back, though, a simple and surefire (for me) way to wake the screen, without needing to force an annoying reboot that seems to be the only solution for many people:
When you experience a blank screen when trying to wake your HTPC, simply press: Windows key + P key, then hit Enter.
You may find that your display magically re-appears.
If you try this keyboard shortcut while your display is working, you can see what it's doing: Win+P invokes Windows built-in Projector setting, which you would normally use when connecting your PC to an external display, like a projector, and need to select how to split or mirror the display. Once the command has been invoked, the Enter key selects the first option, which is just to use your normal display instead of a projector, and you're immediately returned to your Desktop. Note that I've only tested this using Windows 7, so if you use another version of Windows, you'll need to see if there's an equivalent keyboard shortcut for it.
This essentially tricks your computer into refreshing its display output, which (at least for me) is all that is needed to bring the display back to life. I wish there were a more permanent solution to this problem for everyone, but until then, this little keyboard shortcut seems to be the most painless and reliably consistent way to wake my display.
Give it a shot.