Error 1120 and 1172

I'm creating a game and 2 days ago it was working perfectly, now all of a sudden I'm getting the following errors

1172: Definition core:Screen could not be found

1120: Access of undefined property Screen

The code it's referring to is:

import core.Screen;

stop();

(this.parent as Screen).getStateManager().nextState();

I haven't changed this at all in the space of time it was then wasn't working. Can someone give any explanation as to why that may have happened and how to fix it? Thanks to all answers in advance