Thank you for contacting the History Hub.
There are numerous online collections of newspapers, however no single collection is going to be comprehensive and many require paid subscriptions. One free site I'm aware of is https://news.google.com/newspapers however the exact date spans of newspapers they include vary greatly.
Was there a specific newspaper, geographic area, or topic you wanted to research?
One of the first things I'd recommend to anyone looking to find old newspapers is to contact your local public library. Many libraries have collections of their own or have institutional subscriptions to online databases. If you know which newspapers you are interested and those newspapers still exist, you should also contact the newspaper company directly so they can let you know what options exist for researching their back issues.
Chronicling America is free online database developed and maintained by the Library of Congress that provides free access to information about historic newspapers and select digitized newspaper pages.
You may also wish to reach out to the Newspaper and Current Periodicals Reading Room at the Library of Congress, whose staff may be able to direct you to other useful resources. Contact information and a link to Ask-a-Librarian may be found on their website.
All the best of luck on your research!