Max no longer opening main window

[ColmOsiris]

Until the other day, when I've opened Max, it's presented me with a window into which I can drag the files I want to convert. Now that window no longer appears, and I can't figure out how to open it. I've tried the File/Open… menu option, but the files I want to convert (from WAV to FLAC) are greyed out, so I can't convert them. I've tried reinstalling the app, but it didn't help. Many thanks.

Using Mac OS 10.12.6 Sierra.