With just the information you have provided...
"They used to be indexed and searchable on Google, but now they are not."
This sounds like a "Detected PBN".
The sites the links were hosted on are probably penalized by Google. As such they most likely will never show up in the index again.
One more thing you can test....
Checked Bing and other SE's to see if the links are findable. If they are not then it's almost a sure bet the sites hosting the links are under penalty.