That pref stores modification that you made in the about:newtab page. Changes made manually on the about:newtab page are stored in prefs that you can find on the about:config page.

• browser.newtabpage.pinned stores websites that you have pinned to a tile.
• browser.newtabpage.blocked stores websites that you have removed by clicking the X button in a tile.

It is more likely that the new tab page isn't set correctly. You can check the browser.newtab.url pref on the about:config page to see if it is still set to about:newtab

See this article about the New Tab page (about:newtab):

• https://support.mozilla.org/kb/new-tab-page-show-hide-and-customize-top-sites

supitha said

In configuration, browser.newtabpage.blocked and i can't see the newtab page how do i resolve this?? Please help..!

type about:config into your address bar, press Enter. Accept the warning. Search for browser.newtabpage.enabled, right click it, and press Reset.

Then search for browser.newtab.url, right click it, and press Reset.