I can't use restart/shutdown after failing to load El Capitan

I purchased a new iMac last October with El Capitan already installed. It currently runs 10.11.1. I tried to download 10.11.3, it cleared the top bar and the desktop but froze at the dock. I can still use the computer but if I want to switch it off I have to use the power button. I've tried Apple support which got me nowhere the best advice I have is to wait for the next download and hope it corrects the problem! has anyone else experienced this problem?