Often it's a problem with a loose connection or corroded battery terminal, but it can also be just a discharged battery.
The question is too vague to answer in any kind of detail, but it would be good to start with the basics; test the battery. Use a volt meter to see what voltage is present at the battery, then start to go out from there.
If you have voltage at the battery, test voltage at the connectors, then the relay, then the fuse box.
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Not connecting it
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Yes.
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