Horizon mark 111 stitch liner

Hi all…… any one with experience on the horizon stitcher? Am currently trying to figure out a fault which occurred this afternoon. On the screen it reads….. top and bottom knife hitting detection. I can’t do anything with the machine as the fault won’t clear. When I operate the trimmer manually it’s absolutely fine except when I try to open the knifes. That’s the only function that won’t work. This fault appeared whilst running … no jam anywhere or feeder errors. Anyone have any ideas. ??? Thanks.