Based on your engine being a 4-banger, it is likely your water pump impeller is rotted out. The gauge sender and heater core are at the end of a very long and high push. When the pump rots out, the design of the 4cyl cooling system prevents circulation from reaching the heater core and the gauge sender. Your symptoms are classic.
Run through the no heat troubleshooting guide I made and I suspect it will lead you right there.
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