Sandbox is an extension. If an extension earlier in the alphabet runs into trouble, Sandbox, and later alphabet extensions, may not load.
If you close SketchUp, then go into this folder (copy and paste this text into the path field of File Explorer):
Temporarily rename Plugins to something else. Now open SketchUp and see if Sandbox is there. If it is, one of your other extensions may have problems. You could look in the renamed Plugins folder, think about what might be a problem, then close SketchUp, move some extensions back into the newly created Plugins folder, and open SketchUp again. Hopefully you will soon track down the plugin causing the problem.