I have had chrome cast installed on my 8 y.o. LG smart TV for about 12 months without a problem. I could switch between ordinary live TV on the LG to chrome cast using either the LG remote or chrome cast remote. Now I can't revert to normal TV for live to air channels.
Thanks for reaching out. Please try to remove your TV and re add it under the Remote and accessories section and test. Rebooting your Chromecast and plugging it on a different HDMI port on your TV might also help.
Let me know how it goes.