please make a new project, do nothing in it and try to playtest. If that fails, most likely something is damaged within your editor.
in that case, please clear the steam download cache (steam help has instructions on how to do that), then deinstall and reinstall the editor through steam.
After that make another project and playtest again.
If even the reinstall fails, report here - there are a few more possibilities but it will be more difficult to find them.