My 2012 Nissan Versa will not crank, what is the issue??

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Drove my car to get gas, came back out to leave it wouldnt crank, took to a mechanic shop, they replaced the ignition switch, and it drove for 5 days, and now the same thing happened, it wont crank again, and another mechanic is not understanding, they have tested my starter, testes fuel pump, tested fuses, and still dont know why it wont crank!!! Can someone help me please!!