If you mean the resettable one, it resets if the battery voltage gets too low. Have you had trouble starting it recently? If so, the voltage might have dipped low enough to reset the mileage.
The one that shows how far the car has gone since new should not reset itself, ever. If it does, it is faulty and you need a new speedo.
Mine has done this a few times too. It seems to have started since i swapped my Sony head unit for an Alpine one. I have a small sub and amp too but these have been in the car for at least 18 months.
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