Really noob question

When i crop a video it does not resize to fit the screen it just keeps the cropped bit and gets rid of the rest but i don't remember it ever doing this before normally i crop the video and it resizes to fit ??? What am i doing wrong never had this problem before so i don't know whats going on ?

I think what you're looking to do is re-scaling (zooming), not cropping.

Under the Transform section of the Inspector, you'll see the Scale slider. Just increase it above 100% to "zoom" in on the area you want to fill the frame. If it's not in the center of your original frame, you can then use the Position tool to perhaps zoom into one corner or another.

Sounds like you are in trim mode. The onscreen cropping tool has 3 modes, trim, crop and Ken Burns. Trim will "trim" the edges without rescaling. Crop will maintain aspect ratio and re-scale.