Not a failing hard drive! Actually, it might be - I have no way of knowing for sure, but I've been told this by Apple Care but it wasn't the case... TWICE. I'm on MacOS Sierra version 10.12.6. Try clearing cache files first - seriously! Go > Go to > /var/folders > find any folders except for zz (should be a couple or a few folders besides zz, such as l7, 0b, etc.) > then go into the folders until you get to the O, C, and T folders and clear them. Then empty your recycle bin and restart. You should see your app icons again and be able to open them.