Did you try pushing it back in? Look inside the top of the case to see if something is blocking the slider from moving. Did you install an aftermarket PSU, maybe the metal tabs got folded too far in when that was done. It should move out and snap back in by a retrurn spring with the side off. If something is keeping the cover from closing fully like wires across the top of the heatsink instead of in the clips or under the fan then that could do it also. Check for bent latch tabs on top inside edge of the cover. If it was dropped or fell over one of them could be bent.