It started a few months ago and it's really annoying… Help please?

Make it easy; I'm a personal dummy. : /

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Answer:
That’s controlled with the power settings, change the settings if you want it to stay on

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Answer:
Yea what he stated stated is true the battery could be going out, but when you turn it back on does it state restarting from hibernation or something similar? If it does right click on your battery bar in the taskbar(lowerright corner) click adjust power properties see when the laptop goes into standby or hibernation
Then go into the advanced tab
There will be a section called power buttons Find the one that states when i close the lid of my laptop
if you want it to do nothing click do nothing other wise it will automatically standby or hibernate which can be very annoying if your a power user
Hope this helps if not Change your battery and test the ac power, but the ac power check can give you false info because your ac power won’t go into standy when you close the lid. so check the first things then the battery my laptop is over 4 years old and the battery if fine still gets a 2 hour charge

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Answer:
So, your computer seams to have a battery that is not holding it's charge anymore.

You sound like oyu have had this computer for a while now and batteries have a n average life of 12 months -18 months.

Some last longer, some go out in less than three months.

To test my theory, plug the laptop into the ac unit without the battery in. then while still plugged into the wall, close the computer. the laptop should go inot hybernation, but not turn off.

This is proof that your battery is going, but luckily a new one shouldn't cost more than $75

Good luck!