The headphone (or 'audio out') sockets on TVs (as well as on most other devices with headphone sockets) have a switch built into them which mutes the internal speakers when a jack plug is inserted.
That 'switch' is actually nothing more than a springy piece of metal, which is meant to bounce back into the 'on' position when the jack plug is removed. However, if the jack plug is left inserted over a long period of time, the metal can lose some of its springiness, so that it fails to return to the 'on' position.
Sometimes simply inserting and removing the jack plug lots of times (try a dozen or so) is all that's required to get enough springiness back for the internal speakers to be switched on again. If not, it might be necessary to replace the jack socket.
The foregoing assumes that you (or someone else) didn't use a menu function on the telly to switch off the internal speakers when you added the sound bar. If you think that may have happened, consult the set's instruction manual to find out how to reverse the procedure (or tell us the make and model, so that we can look for you).