This led to a solution - apparently this is due to an update in Rmarkdown which broke something in distill's ability to build the landing page of websites. Installing the dev version of distill from GitHub will fix this, and now the index.html page will build properly.
Just FYI- the take-home message - anyone updating their rmarkdown and using {distill} - you can get a weird bug where
- your website appears to build fine (no errors) but
- your Viewer tab will be blank after the build (normally shows index.html, your landing/home page)
- after you push your website, your index.html page appears blank (other pages load fine)
but this is fixable by updating updating to the dev version of distill.
Not an obvious fix, but it works.
See Distill site cannot rename index.html anymore - #2 by dataning