The Classic Catalog (which some still referred to by its predecessor's name, CUNY+) functioned well for eighteen years (since 2002). But during Summer 2020, the Classic Catalog was decommissioned, as the CUNY Office of Library Services replaced and upgraded the system behind the catalog.
The Classic Catalog has been replaced by OneSearch as of August 3, 2020, which many people have already become familiar with. It is a powerful discovery tool that can search across multiple platforms and retrieve records for books, articles, ebooks, videos, and more, all in one interface.
Aside from connecting directly to OneSearch, you can also use the search box on the Library website.
Basic searching with OneSearch video tutorial.
Advanced searching with OneSearch video tutorial.
Using citation tools in OneSearch video tutorial.
For more suggestions about how to best utilize this powerful discovery tool, check out this excellent guide OneSearch: Where to Begin.
For a thorough literature search on a specialized topic, consult specialized subject databases rather than just OneSearch alone. Check the Library's research guides for suggested databases.