I must've knocked my guitar or something 'cause the hole where you put the lead in is loose and so the lead can move around. How do I fix this? Cheers.

There is a nut there that needs tightening. You can use a nutdriver.
Unscrew whatever's holding in the jack. If you just turn it from the outside, you can cause wires to break.
laughed my ass off at that "loose input hole" ha

seriously though just unscrew the metal thing holding the input in and then you can just tighten it with your hands.