Google the term "pole shift". That's one theory that has been proposed.
A regular african elephant can easily eat 3-4 hundred pounds of grass and bark a day. A mammoth can be expected to eat even more.
I saw a picture of an excavation site for on of the mammoths once. All you see is ice. Then, near the horizon, you see alot of frozen water. That's it. No trees, no waving fields of grass, no low lying scrub plants, nothing.
And there were TENS OF THOUSANDS of the critters up there. Along with giant rhinos and a bunch of other very large mammals.
The gold miners in Alaska and northern Canada found stuff. Vast areas of smashed and crushed bones. Few of the bones were articulated (like your bones would be connected if you fell down and died and decomposed and weren't eaten.)
The majority of the stuff (they call muck) is like it went through a blender crushed and smashed bones, trees, river rock, all mixed together, and flash frozen.
Whatever happened, it WAS NOT a nice gentle warming where the ice caps melted and gradually receded. It was much more cataclysmic.
Clone the beast!
I wanna eat mammoth meat on Mondays!