It sounds like your dust/splash shield.
I'm sure there are 15 other names for it also.
It is a plastic piece that covers up the bottom opening of the engine bay.
Typically they are just attached by 5-10 screws or fasteners.
The purpose is to keep dirt and liquids coming up into the engine bay.
Although not crucial, your car will continue to operate without it, you will have more dirt accumulate in the engine bay which can cause issues over time.
http://www.courtesyparts.com is a great place to find Nissan OEM replacement parts. They are typically less expensive then other dealerships because they do so much parts volume they give very good prices. The shipping will sometimes negate the savings depending on the price at your local dealer though.
I'm having trouble locating the shield on the diagrams for the 2007 Sentra, sometimes the parts are in a strange spot or not visible.
So I'd suggest you call Courtesy or your local dealer's parts department and they will be able to give you the price of the correct part based on your VIN.
Another option is to go to a local junk yard and grab one off of a wrecked Sentra. It will be cheaper than a new one and pretty easy to clean up before putting on your vehicle.
http://www.picknpull.com is a good one if they are in your area as they have a search tool online to see if there are any appropriate year vehicles on the yard and when they arrived (the more recent the better chance it has your part).