Can't open hood from inside; latch has no tension

Normally, the cable snaps off from behind the cabin release lever. In that case you can pull the cable end with Locking Grips. If it has broken at the hood end, and they have a history of it, there are videos in the link to get in through the grill.

https://www.google.com/search?q=2012+Nissan+Murano+hood+won%27t+open+after+pulling+hood+release

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It sounds to me that your cable which travels fron the hood latch to the interior handle is binding or sticking. This cable can be replaced, but first check for kinks where it travels thru the frame of the car. Lubricate both the latch assembly and the cable from both ends. With the hood open, the latch inside the car should move freely back and forth and you should be able to feel the spring tension in the latch assembly. The cable is very similar to brake or shoft cables on a bicycle.

The cable is probably broken and not connected to the latch. There are two latches, a primary which the cable is connected, and the secondary or safety.
You'll have to try to access the latch from under the car going up between the bumper and the radiator. You may need a body shop to do it.

Have someone push down on the hood over the hood latch area while you pull the release, that may work. Once it is open lube everything up with white Lithium grease also known as "LUBRIPLATE:"

The easiest solution would be to find the end of the cable at the release handle. If it's the handle that broke, the cable end may still be there. If so, grab it with a pair of pliers, and pull. Make sure your are pulling on the inside cable, not the black outer casing.

The other option is to crawl under the front bumper and see if you can get your hand on the release arm. The cable is attached to it. Have a helper push down slightly on the hood, over the latch. That will help reduce the spring tension on the part you need to pull on.

close hood and lift up on hood to see if it stays closed.
if not, check inside car to see if latch(that you pull) is in the correct position. you can tell when hood is closed that there will be some tension on latch.
check under hood, where hood closes on latch of hood, is tight and secure(no loose bolts)
also check cable if broken.
only those things can cause hood to pop open.
if all visual inspections check out and still continues to open while driving then the latch (the mechanism that hold the hood down) is bad. vibrating while driving will cause it to open.

u can't get at the latch unless the hood is open, kinda a catch 22, try this, have someone push down on the hood right over the center in front, that is where the latch is, this unloads the spring tension on the latch and many times it will allow u to pop the hood, then get some WD40 and sopray the latch

You can pop out the intake plastic grid and access the hood lock mechanism from there

The latch is stuck in the open position. I always found white lithium grease worked best on those latches but any good lubricant will work. Try moving the latch manually with a screwdriver. After applying the lubricant move the latch a few times. You should see the tension return to the release, if it doesn't you have a broken release cable.