This is due to your device id changing and not being listed in the driver file -
First - make sure you have the updated USB drivers from here:
Google USB Driver | Android Developers
If you have the updated usb driver and its still not working, then it is very likely the device id issue that I mentioned at first. To fix this you need to edit the drivers .inf file to update the device id.
Step one: With the tablet plugged into your computer, go to device manager in windows and find the nexus 7 on the list, right click on it, and click "Properties". Go to the "Details" tab, then in the "Property" drop down box, select "Hardware Ids", in the box below the dropdown you will see some values. Right click on the first value (it will be something like "USB\VID_18D1&PID_D002&REV_0228
") and click copy.
Step two: locate the directory where you installed the usb drivers and find the file called "android_winusb.inf", open that file, it should open in Notepad. In that file there are two entries for the Google Nexus 7 that look something like this:
;Google Nexus 7
%SingleBootLoaderInterface% = USB_Install, USB\VID_18D1&PID_4E40
%CompositeAdbInterface% = USB_Install, USB\VID_18D1&PID_4E42&MI_01
%CompositeAdbInterface% = USB_Install, USB\VID_18D1&PID_4E44&MI_01
You're going to add an additional line to both of those sections of the file including the device id you copied in step one the line you add should look like this:
%SingleBootLoaderInterface% = USB_Install, [paste the value you copied in step one here]
So for me, my device id is USB\VID_18D1&PID_D002&REV_0228 so when I get done both of the nexus 7 sections of the file will look like this:
;Google Nexus 7
%SingleBootLoaderInterface% = USB_Install, USB\VID_18D1&PID_4E40
%CompositeAdbInterface% = USB_Install, USB\VID_18D1&PID_4E42&MI_01
%CompositeAdbInterface% = USB_Install, USB\VID_18D1&PID_4E44&MI_01
%SingleBootLoaderInterface% = USB_Install, USB\VID_18D1&PID_D002&REV_0228
Then save the file, go back to device manager, right click on your nexus 7 and click "update driver", locate the folder where you have the android_winusb.inf file and install. Should now work.
Let me know.