Check the ACPI settings in BIOS: Power Management Settings : BIOS for Beginners
Once the BIOS is set correctly with all ACPI-compliant hardware, the next thing to focus on is display adapter. Make sure you have the latest driver from the Support downloads webpage for the computer maker and/or adapter maker. Check all available drivers back to and including Vista. You can roll them back on Driver tab in Device Manager as well as "Look at a list" of available installed drivers.
Next check the Performance log to see if it is logging hangs at time of suspend: Win7 troubleshooting
I'd especially resolve all repeat errors in logs listed, run a clean boot to see if it will suspend, and clean up the startup list.
If this gets nowhere, use the tool used by the pros to troubleshoot these issues: Trace Windows 7 boot/shutdown/hibernate/standby/resume issues - MSFN