Start by removing HDMI cables from the TV and see if the TV will turn on and off without any issue.
If this resolves the issue with the TV turning on, I would look to replace the HDMI cables, however if the issue is persistent its worth performing a factory reset via the TV menu.
After performing a factory reset if the fault is still present its likely to be a hardware fault on the TV itself, at which point its work arranging the TV to be inspected by a service centre.
Should you feel the fault is with the TV, if you provide your postcode I can recommend a suitable TV repair centre near you.
Sorry to hear the issue has reoccurred, I would recommend you contact McDowell Electrical on(###) ###-####6558 or Cregagh Electrical on 028 90731627, both serve your area, and give you a good price for repair.