Start by checking your system everywhere. Are you sure that you have connected everything everywhere ? Did you drain the hot water heater and remove the line ? Have you reconnected it ?
Your pump has a pressure sensitive switch on it that tells it when to turn on and off. This can need adjustment occasionally.
When you are sure that everything is open or closed and attached or isolated, fill your water tank up all the way. Give the water a little time to make its way into the system. If you have any air traps or loops in the system, you may need to prime some areas.
Then turn the pump on and let it pump everything through. If everything is fine and it still won't turn off, then you need to adjust the pressure switch. They can get cruddy and stiff with age. Look for a manual online for the pump and adjust it until it is working well. The switch is usually up at the top end near the intake.