Home Page Deindexed overnight?

Hi, 

Hope you guys can help. I run an e-commerce site https://alloywheels.com Last night our home page (and a few other pages, but not all) were de-indexed by Google. The site has been ranking (UK) for years in P1 for the "alloy wheels" keyword and on the whole been running very successfully. However recently I have noticed from fluctuation on the "alloy wheels" keyword, dropping to P3 then P5 then back to P3, but this morning I noticed we were not even ranking on the first page.

When I check inside Search Console there are no messages or warnings but the "/" page was de-indexed. There were a few other key pages that were also de-indexed. I have request reindexing and they have come back, P7 for the home page for "alloy wheels"

The only thing I have changed was I realised yesterday there was no robots.txt on the site and was being recommended by web.dev to add one, so I did. It was just an allow all:

I ran tests on the robots.txt before it was uploaded and it all came green. I have removed the robots.txt for now.

Has anybody seen anything like this before? With the recent ranking fluctuation I am not sure whether it is to do with that, the robots.txt or something different altogether?

Thanks in advance, 

   James