What's the reason to be downgraded even without backlink spamming?

My website was well ranked generally, I mean not on page one, but satisfactory generally. But suddenly it doesn't even show up when the whole name (without spaces) without the tld is searched! I mean google has forgotten it altogether. There are no backlinks at all. There were a few, and I disavowed them by adding them in a txt file in search console. What could be a possible reason please? The domain is 5 years old, and was alright until about 6 months ago I think. I did some Facebook advertising; could that have had any effect? Any advice would be really helpful. Thank you.