My entire website has been completely removed from Google's index last night.
How do you know this? Are all pages missing in GSC? Are the pages deindexed, or just missing from SERPs? What about other SEs?
If the site has been removed without warning or notice, check first for pilot-error and unauthorised access (hacking): obviously you or anyone else with access can block some or all pages in GSC, but anyone with server-access can also do it in the HEAD section of individual pages, globally in the root-directory .htaccess file, or locally in subdirectory .htaccess files. Check all of these, if you haven't already. However, changed individual pages wouldn't usually disappear all at once (when was your last googlebot crawl, and what was crawled?).
The site could be deindexed without (or before you receive) communication from Google if there are incontravertible DCMA violations or for posting - do you allow UGC? - illegal or defamatory content. If that has happened, you can expect to receive an email within a day or two, but from the perspective of someone who has been on the receiving end of and reported many DCMA and trade mark infringements, I'd be surprised if anyone reporting either of these had been instantly and suddenly effective in your case. Illegal or defamatory content is about all I can think of that would be acted on at once and/or without notice.
Most likely, I think, it will turm out to be a temporary glitch, but these can often go on for weeks (Google removed my UK-based .com site from UK DCs and results for over a month as part of a location-based results experiments some years ago). For now, going forensic on your server logs will probably tell you more than anyone here can.
edit: corrected typo
