The answer to 'why is my septic tank no hot water' is that a problem with your septic system is unlikely to be the cause of no hot water. The septic tank and the home's hot water system are separate systems, so issues with one would not typically affect the other.
The most common reasons for no hot water are problems with the water heater itself, such as a malfunctioning heating element, thermostat, or other internal component. This would require troubleshooting and potentially repairing or replacing the water heater. You may also have issues with the water supply to the heater or the home's plumbing that could cause no hot water. These are all problems that would need to be addressed by a professional plumber, as they require specialized knowledge and equipment to diagnose and fix properly.