Stick it in some dechlorinated water, add some aquarium water (for food) and wait. If this one dried out, just grab another one.
If it doesn't work that way, go back to the pond, try to reach down and carefully grab more of the plant near the soil. If there are runners coming from it, grab those as well, although you may have to snip those. If you do have to cut runners, try to keep 4-6" of runners intact attached to the plant.
It may take a few tries, but as long as it's kept wet, and you have the right type of plant, you should be ok to grow it.
I wouldn't just stick it in the sand without roots; instead, I'd try to get some roots going first in water before I added it to the aquarium.
Also, make sure to sterilize the plant and water as there may be unwanted organisms tagging along.
