How is the all-in-one connected to your computer? What OS are you using? Did you install the full software, basic drivers or the OS drivers? Are you starting your scan from the computer or the buttons on the all-in-one? Is this a newly installed printer or one that has been used several times with this computer?
The Photosmart C7180 can be used with a USB connection, wired ethernet network, wireless network, bluetooth or PictBridge. For a Mac OS, only the USB connection will work.
If you need the drivers, see http://h10025.www1.hp.com/ewfrf/wc/softwareCategory?cc=us&dlc=en&lc=en&product=1153754&
and select your OS.
If you connected the all-in-one to the computer via a USB connection before installing the software, you may have an OS driver that does not include the scanner driver. Disconnect the USB cable then uninstall the driver. Then install the HP driver. If you do not install the full software, you will not be able to use the buttons on the scanner to launch a scan. You'll have to start the scan from the computer.
If you have the full software, the HP Solution Center is installed. This includes a monitoring program that should run when you start your computer. This listens for the press of a button on the scanner and launches the appropriate part of the HP software. However, you need to set the button destinations in this software first. I recommend manually scanning at least once to set the appropriate resolution and destination folder for your scans. Then in the Solution Center, click Settings, Scan Settings, Button Settings and then Front Panel Scan Buttons. Set these based on your default settings.
If this all-in-one has been working previously, check your connection. Scanning requires bi-directional communication with the computer and can fail even when the printer works. Try a new USB cable and a different port, if you are using that connection. If you are on a network connection, check that the firewall isn't blocking communication. (Temporarily disable the firewall to check.) In Windows, check the Devices and Printers (Win7's name) control panel and check if the scanner is a listed device. Right click on it and check the properties and status. (All-in-ones are often listed twice - as a printer and as a scanner.) HP offers a diagnostic program, http://h10025.www1.hp.com/ewfrf/wc/doclink?lc=en&cc=us&product=1153754&tmp_docname=c02114802
, to check for issues with the scanner.
Try manually scanning with another program than the HP software. Windows includes some options that will acquire scans. XP, Vista and Win7 all include Paint (File > Import from Camera or Scanner > New Scan). Windows7 (and some versions of Vista) include Windows Fax and Scan (click New Scan). If you have the Microsoft Live software, you also have Windows Photo Gallery (works like Paint).
If you are connect the all-in-one to a USB print server, the scanner probably won't work. Try a different type of connection.
If you are using the all-in-one on a network, make sure that the network is up and the printer has an IP address. I've seen routers lose their connection for various reasons. (Rebooting both the modem and router sometimes cures the problem.)
Please add more information on your specific scanner problem and I'll try to provide specific suggestions.
I hope this helps.