MacBook Unibody 7,1 fan works but MB doesn’t turn on.

Hi, after thermal paste replacement my MacBook doesn’t turn on. The fan works and a white light on the front too at the beginning but then the light turn off and on like a car indicator but slower. It doesn’t do the typical optical drive sound when I press the power button but the fan start working but nothing happens.

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Do you see any change in the display? Does the screen appear to change in its grey color or have any glow or image?

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Don't know what happened and I don't know what I did but when was going to give up I armed averything and tried for the last time to turn it on and it did it! But now when it turns on in the screen where I have to choose the user, the back screen is gray like the color when it is turning on. Before it was my wallpaper. Any solution? I think I turned it on safe mode or something like that.

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