Amazon discontinued Kindlegen yesterday. It is no longer available for download.
What happens now if we want to compile to .mobi and haven’t already downloaded and installed Kindlegen? 
You use Kindle Previewer available from the Amazon site.
Even if you already have Kindlegen, you should strongly consider downloading Kindle Previewer and replacing it. Kindlegen is no longer available as a separate download, but (a much newer) version is bundled with Previewer and Scrivener 3.x will detect and use it.
Thank you both. I will go download that now. 
I don’t know if this is directly related, but I cannot now compile to Kindle format – it seems to work well and Kindle Previewer opens up and shows me my book, but then it crashes with an error dialog box that I cannot read. It doesn’t save the file, so I’m left with no output.
I downloaded Kindle Previewer and now Compile crashes every time I try to make a Kindle ready copy of my novel.
Can anybody help me fix the issue?
You have to export to ePub. I do pre-flight testing in Sigil and then check it again in Kindle Previewer. Works fine.