Why is this spider wound in such a dense web?
From the look of the spider outline, it looks to have short legs and is dark. A yellow sac spider is quite light-coloured and has long legs. Yellow sacs and clubionids will sometimes make themselves a sac for resting or eating or molting or laying eggs. Yours is probably a jumping spider, and here is a youtube timelapse video of a jumping spider laying eggs while inside its own protective sac:
https://www.youtube.com/watch?v=yoqMNX0RWuc