An easy way of getting Vannilla Sandbox to work is when on the menu, goto Addons > Disable All (This will disable your addons, not remove them) and then start a Singleplayer game. Essentially, what is happening here is that so many addons (.gma files in console) are trying to load up, you can see it in console it looks a bit like this:
And basically all these addons are loading up and there are so many that it crashes. Also the addons could have conflicting hooks or code that are overwriting or interfering with eachother.
I have a feeling your addons could be the problem so as the people said above, Try removing some of your addons or disabling them. If that don't work. Re-install GMOD if it makes any difference.