A furnace limit-switch reset is probably needed if your blower will not shut off even with the thermostat in the off position
The filter is probably dirty and causing the furnace to overheat, thus tripping the limit switch. Newer models have a reset button (see pic) tht can be pushed in to reset. Older models may need time to cool, then they will self-reset. Other causes could be various air flow issues like furniture blocking the return grill or too many supply registers closed.
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