Either the electronic temperature sensor that controls room temperature has gone bad ....... or the control module with the buttons on it has gone bad. Repair costs in the USA or the EU will run $200 to $300.
It is not always easy to get parts for the Samsung units...for that reason, and the fact that repairs can cost more than a new unit in many cases, some appliance
shops will not accept them for repairs.
The life of the fan motor is in the 5 to 7 year range. That will be the next to go, cost to repair that will be in the $400 range.
My advice is to replace the unit. I am sorry to give you bad news, but any advice to attempt repairs would be a disservice to you. Most if not all of these air conditioners made in Asia are not repairable at US labor rates of $100 an hour., especially with 5 years use on them.
Beyond this range of issues, virtually all AC units on the market will ice the coil if run below 70F at night. You were very lucky with this one. Some of the AC units on the market will not let you set the temperature below 70 or 72F...
*** it will be a very good idea to check that on the next unit you buy***
Let me know what you think, we can go from there.