any error messages?
the 3 common reasons are:
1.) incompatible graphics card driver, i.e. OpenGL support not compliant => update
2.) incompatible plugins/extensions => udpate/disable*
3.) Win: wrong or missing prerequisites (= MS runtime libraries) => install SU again by “r-click > Run as Admin” or install the libs manually one by one (point #5).
*disable: start SU from the command line with the “\DisableRubyAPI” flag… or rename/move the plugins folder.