The answer to 'thermostat not turning on' is that there are several common causes, including a dead battery, loose wiring, or a faulty thermostat. If your thermostat is not powering on, the first step is to check the batteries and replace them if needed. You'll also want to inspect the wiring connections to ensure they are tight and secure. If those basic troubleshooting steps don't resolve the issue, the thermostat itself may be malfunctioning and need to be replaced.
In most cases, replacing batteries or checking wiring is a simple DIY task. However, if you're not comfortable working with electrical components or the problem persists, it's best to call in a professional HVAC technician to diagnose and repair the issue. They have the expertise to properly identify and fix any underlying problems with your thermostat or HVAC system.