First try a different data cable and a different port. If you use a USB hub or print server, try a direct connection. Check that a known working device can connect to the computer. Check the device manager and power saving settings. What message do you see in the Devices and Printers window (Printers and Faxes for Win XP) for the MP210? What error messages do you for printing and scanning?
For Windows 7, Start > Control Panel > Hardware and Sound > Device Manager. Look for a triangle with a yellow exclamation point or a red x at the bottom of the list at Universal Serial Bus Controllers. You can try updating the drivers for the USB controller or uninstalling them and then rebooting the machine. (The computer should find and install new drivers automatically. However, you should also check in Windows Update for them (check for new updates - optional and important).) With some computers this could be the result of static at the USB ports. Disconnect all of the external devices and the power cord (computer and the outlet). For a laptop, also remove the battery. Then press and hold the power button for 30 seconds. Then put the battery back in (if applicable) and reconnect all of your cables.
For the power savings settings, again for Windows 7, right click on the desktop (anywhere there are no icons or running programs). Select Personalize. Then click on Screen Saver then choose Change Plan Settings. Next click on Change Plan Settings (for the active plan). Then click on Advanced Plan Settings. Scroll down to USB selective suspend.
Did you install the Canon driver, full software or the OS driver? Can you print any documents? (Sometimes the scanner will not respond but the printer still works.) If you have documents in the print spooler, you may need to stop and restart the spooler. (One way to do this is to launch the Command Prompt window (run as administrator for Vista or Windows 7). Start > All Programs > Accessories > right click on Command Prompt and choose Run As Administrator. Type "net stop spooler" without the quotes followed by enter. On the next line, type "net start spooler" again without the quotes and press enter. Now restart the computer.
If the printer works, but the scan buttons do not. If you connected the USB cable before installing the Canon driver, you
have a generic driver that may not run the scanner. Uninstall the
driver, unplug the data cable and then reboot the computer. Then install the full featured software (if available) from Canon http://www.usa.canon.com/cusa/support/consumer/printers_multifunction/pixma_mp_series/pixma_mp210?selectedName=Specifications#DriversAndSoftware
. Next try to scan from the computer. You need to set up the full software with destinations for the scanner buttons and the monitoring program must be running. (If you use the basic driver, the scan buttons will not work. In Windows XP, Vista or 7, you can acquire scans with Paint. File > Import from Camera or Scanner > New Scan. Vista comes with Photo Gallery that can also do this (it can be downloaded for XP and Win7). Vista and Windows 7 also include Windows Fax and Scan to acquire scans.
If the unit still fails to work at the current computer, try connecting it to another (with known working USB ports). If it still fails, then the USB port has failed in the all-in-one. This could be another static issue. With the printer on, unplug the cables (data and power from the printer, outlet and the computer). Press and hold the power button for 30 seconds to a minute. If this doesn't help, if the printer is under warranty ask Canon to repair it.
I hope this helps.