Cutworms, if at or near the soil line, are the most common culprits. Any other soil residents like sow bugs and/or earwigs would be in large groups and you'd see evidence of them.

Beyond that, but usually a little higher on the stem, are mice and/or rats.

Dave

Turns out it was a gopher. Last night I lost the pumpkin plant. When I dug I found a tunnel. Damn gopher. I put a gopher trap there and next year ill have to use a gopher basket.

Smoke bombs (easy directions) work well too on gophers...goph for it.