Turned on heater this morning and no heat. Checked water levels and they are ok.

replace thermostat before you do anything else.. If it is stuck open there will be not heat

Air lock in heater matrix, or matrix clogged with sediment. Is water level ok, is coolant clean?

Heater controls malfunction.

Water pump faulty/belt slack.

low level of heat could be thermostat jammed open.

Blower motor inoperative/slow?

Make sure both lines going to the heater core are hot and about the same temperature. If the heater core is ok ...I would look at the temperature blend door motor on the heater box. Good Luck

check the water pump, might be leaking, or the radiator.

Seems like you have checked everything. Now you can basically pinpoint the heatcore as the problem. They do go bad. No heat in car, this would be your problem

If your "Check Engine" or "Service Engine Soon" light is on, you need to have that checked out first.
If not, the next thing you should check is your engine coolant level. (Antifreeze) If the coolant level is low, the engine coolant does not circulate through the heater core and the the heater doesn't work. The coolant level could be low for a number of reasons including a leaky hose, water pump, or head gasket. In any case, if it is found to be low, you need to find out why.
If the coolant level is OK then the thermostat function needs to be checked to make sure that the engine is reaching operating temperature.
If all of the above checks out OK, then the heater control valve and/or the blend air door in the HVAC housing needs to be checked for proper operation. (NOTE: some cars are not equipped with a heater control valve)

When driving down highway only, it overheats? When running steady, I take it you mean, sitting idling? If it's only overheating at highway speeds, and all else has been checked out & is o.k., I'd take a close look at rad hoses for soft spots (easy to squeeze) that may be causing hoses to collapse at highway speeds & restricting flow of cooling system. Another thing to try is, turn your heater on high fan & high temperature when it starts to heat up & see if temperature gauge goes down (cooler) at all. If it does, then rad not doing it's job properly.

you might need to get the water level sensor checked, it may be faulty

check water level in rad if ok check thermostat or couldbe a plugged heater core.also check for vacum to heater door is working