Computer frozen when changing from finite world to infinite world

I imported one of my old finite worlds. I hit make infinite, it went to a loading screen saying "making backup" so I browsed chrome for a little, switched back to MC and it was still in the same spot, it was stuttering music, wasn't able to move the mouse, ctrl alt delete wasn't working, alt shift esc wasn't working alt F4 wasn't working. I had to push the reboot button on my PC to get it to restart.