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I have a garmin nuvi 750 that has the top button for on and off is broken
how can I repair this problem because the screen is locked

You don't have to replace the switch, a good shortcut is to just mimick the functionality of the switch. It may work for you to put it on permanently unlocked.
If you can open the GPS, try to find out if the switch doesn't make contact with the slider, or if the switch is broken. If the switch is broken, try to either short the pins involved in keeping the GPS off hold or disconnect one of the pins, depending on what electrical condition the switch has to be in to get the GPS to be off hold.