Hi Craig ,
couple things , I would make sure you are on the latest approved graphics drivers for your card which can be found here : http://www.solidworks.com/sw/support/videocardtesting.html
if that doesnt work I would try exporting your custom settings and resetting your registry (making sure you back it up) and see if the issue is fixed. if it is you should be able to import your settings to get your customized interface back. instructions for this can be found here : http://www.javelin-tech.com/blog/2011/09/resetting-solidworks-windows-registry/
I just ran into this. I was making my parts in context of a larger assembly and must have clicked the "No External References" button in the top ribbon. Turned that off and back to normal
This issue has started for me shortly after upgrading to 2018 SP01. When I ctrl select multiple objects, nothing pops up. No Quick Mates and no Context Toolbar. It works fine in part mode, just not in assembly mode. I followed Scott's advice. I have the latest driver, and resetting the registry files did nothing. Any help on this would be greatly appreciated.