there were 2 updates this month, one called "spam update" (which finished a few days ago), and one called "core update" which is still going...
You need to see what these 2 updates targeted and then you'll know whether there is still hope for your site:
https://status.search.google.com/products/rGHU1u87FJnkP6W2GwMi/history
From what I've heard, one of the updates targeted expired domains that are using niches that are not closely related to the previous niche. So, if your site falls into this category there's probably no way to bring the rankings back, in which case I would abandon the project...
The other update seems to have targeted AI content, so if you had (manually non-edited) AI content and it's THIS content that got deindexed then my thinking is that you can still make use of your site by manually editing the deindexed / deranked content, or simply deleting it and focusing only on content that's been manually edited...
Figure out on your own where the problem is (or look up the latest google updates on the forum, plenty of threads about this already) and fix accordingly and you might be able to salvage the site