First things first. Before you do anything you need to walk over to the breaker and make sure that nothing is turned off. Last thing you want is to like like an idiot who didn’t check the power.
One simple way to your thermostat is a simple old fashioned dusting. Use a small brush or something with a cloth on it and dust it as best you can. Dirt and dust are usually what cause most from not working so make sure its super clean when you check it.
One thing that most people over look is the location of their thermostat. You would be surprised how often peoples break down because of direct sunlight or over heated areas. If the sun is hitting is all day, its time to move it to a new location.
Most common problem is the balance. Make sure your thermostat is perfectly balanced, you can use a leveling device to fix this issue.
If you have done all the above steps then check also the batteries, most use AA batteries but there are also some newer models that take AAA batteries too. You should get in the habit of checking the batteries once a month at the bare minimum.
Here is a simple and easy video to learn how you can change the batteries safely and without any bodily danger to you or anyone close by.