There are a couple of issues that can cause this. 1) Did you install the full software before you connected the USB cable (if you are using that connection)? If not, did you uninstall the generic software and remove the data cable before installing the software? Try a new data cable and a direct connection if you are using a print server or USB hub. Scanners require bi-directional communication and will fail even when the printer works.
Are you launching the scans from the computer or the scanner? Try launching the scans from the computer first. Remember you need to set the destination button defaults in the HP software and the monitor program needs to be running to see those button presses. Try scanning with Windows Fax and Scan (New Scan) or Paint (File > Import from Camera or Scanner > New Scan).
Navigate to http://h10025.www1.hp.com/ewfrf/wc/softwareCategory?cc=us&lc=en&dlc=en&product=439488
and select your OS version (32- or 64-bit Win7) and click Next. Then look in the section Utility - Diagnostic Tools. Download and run the HP Scan Diagnostic Utility. Also check if you need one of the Update patches (there is one for scan files going missing).
If you are using the unit as a networked printer, also try a new data cable. Make sure the router is not dropping packets and that your firewall isn't blocking the incoming signal from the scanner. (Try scanning with the firewall temporarily disabled.)
Check if the unit has a scanner lock and unlock the transport lock if it is in the incorrect position. Check if the lights come on and the unit moves when you try to launch a scan. If the lights aren't coming on or the scan unit doesn't move, the unit probably needs service.
I hope this helps.