Assuming that in the past you were able to turn it up without the TV shutting off I would say that either the TV speakers are damaged and causing the TV to shut down or the amplifier driving them has a problem.
Depending on the age and price of the TV using a soundbar with an optical connection might be cheaper than paying a tech to repair the TV. If you want to take a stab at it yourself
www.shopjimmy.com
sells replacement boards and are quite helpful.