Originally posted by Frybags:

You have to make sure not to touch anything on the loading screen. The first load takes a very long time and it triggers the "not responding" mechanics in Windows if you press any buttons or move your mouse. It could be upwards of 5 minutes but just trust it to finish.

it spends ages updating your worlds files.. this advice above is true as!! (also might want to remove mods first till vanilla works then reinstall if that floats your boat)