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Your powered chair may have either shorted circuitry or a sudden failure in the batteries.
Batteries die after being in-use for years. Has yours had years of usage yet?
If you just replaced the rechargeable batteries in it, try charging it up for at least 6 hours on the charger (regardless of what the charger says about its being fully charged) - sometimes new batteries need a long charge before the first usage.
Shutting Off at power-On can also be a sign of a shorted-out component elsewhere. Look for meal debris loose in the chair's workings, or loose connections/frayed wiring/etc.