Check Control Panel to see if your Win 7 computer recognizes your Nexus 7 as a device.
If it doesn't, you need to reinstall the device drivers. If it's recognized as a device, and Windows Explorer doesn't see it, you probably just need to reset your Nexus 7 's settings to get it recognized.
In Developer Options, make sure USB debugging is checked and turned on. In Storage, click on the 3 bar menu icon in the upper right corner, and click on USB computer connection, and connect as MTP or PTP.
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