If you are using HDMI, then you have a few options: some are free, some costs some money.
1. Pretty much the guarantee one to work, is the most expensive. You purchase an
HDMI Detective. This device maintains an HDMI signal constantly so that your display always sees it upon waking and your video card always sees a display connected when resuming.
2. Cheaper option is a
small repeater that some have reported to fix their sleep/resume issues. Read the user reviews to verify.
3. Free is to review
this post which mentions Denon AVR-specific issues.
4. Block pin #19 on your HDMI cable like so, which disables the hotplug pin on your HDMI cable. This prevents either device from thinking that something's been disconnected and disabling the connection.
If it's not a HDMI handshake issue and you are using Windows/XBMC, you can try XBMCLauncher to shutdown XBMC on entering sleep and restart it upon wake (with an added delay, if necessary). In additon, it can launch external apps (like a HDMI utility).