I closed the tab mid-import, have I lost it?

Your progress is kept. Business Profiles you uploaded stay with the draft, so open Import entities again and carry on. A spreadsheet is different: it never leaves your browser, so you pick the same file again, and your column choices come back once the headers match. A draft is deleted seven days after you started it, so finish inside that week.

We keep the draft, never the spreadsheet, so picking the file again is what a resume needs. The seven days run from when you started the draft, not from when you last touched it.