It won't load into Minecraft it's self it keeps crashing at 51

When I open Minecraft and at the loading screen it loads to 51% then crashes It has been happening for 3 days now and I tried restarting my computer and it did not work, It happened to my friend as well but they had to uninstall Minecraft and reinstall it again, But I don't want to do that I have to lot's of worlds I don't want to lose

That red part at the end of the clip Is Minecraft closing