Thanks for reaching out and I'm sorry to hear that you're having an issue with your Chromecast. Let's sort this out.
A few questions: When did the issue start? Is the Chromecast plugged directly into the TV? If not, where is it plugged into? Has the Chromecast ever worked with this TV? Do you have an Audio-video Receiver (AVR) connected to their TV?
Give these steps a try:
Let me know how it goes.