Lenovo k450e tower can't put case back on. Not sure how it slides back on?

Place the case on a flat surface with the open side up. Inspect the panel to see if the "catches" are bent out of shape (if you opened the case correctly, they should be fine). Straighten them with if they are significantly bent. Locate the lock tab on the case frame that slides into a small slot on the rear of the panel. Straighten it if it is bent.
Notice how the catches on the top and bottom match up to slots on the case frame. Lay the panel onto the frame, with the catches into the slots (you will probably have to slide the panel 1/2 to 1 inch to close the case). Use gentle pressure to slide the panel into position.