Not sure what happened here. I play Skyrim with many mods, but I had installed no new mods when this problem appeared. In fact, I was taking a fifteen minute break, and when I started Skyrim again, it kept opening in windowed mode. I looked up the problem online and was advised to delete my SkyrimPrefs.ini and Skyrim.ini files and start the game, apparently because this would put it into default mode. Unfortunately, it only started in a smaller windowed screen. Upon restoring the ini files, it returned to the windowed mode that only filled up half my computer. In addition, when I try to load any saves, it stops responding in the loading screen. This is a problem I have never experienced, and only began when it suddenly started running in windowed mode. Any way to get back Skyrim fullscreen?

Just use the stock launcher, and uncheck Windowed mode in there. Once you do that, you should be able to use the SKSE launcher again and be full screen.

Managed to fix the windowed problem. Took Steam into offline mode and adjusted the settings in options. Unchecked windowed. Unfortunately, now when I load I -still- freeze or CTD immediately in the loading screen.