Hi random person,
Okay, to start with, you can download the Hardware Maintenance Manual for your laptop from Lenovo here.
[link|https://download.lenovo.com/consumer/mobiles_pub/s145_series_hmm_201902.pdf|Lenovo IdeaPad S145 Series Hardware Maintenance Manual|new_window=true]
Start with the section titled "Power system checkout". Then check the status indicators for the AC power adapter and the Power status indicator to see if they tell you anything.
Next, we'll want to start with the basics; is the computer getting power at all? You've hopefully verified the AC adapter during the Power system checkout procedure, so we know that's good.
Next, we want to be sure power is getting to the motherboard from the power plug. Go to the first section of the Removing and replacing FRU section and remove the Base Cover, or the bottom case of the computer. From there you should be able to locate the power jack. Put a voltmeter on DC power and the range should be able to cover at least 20 volts, which you're looking for on the power jack. There should be a single pin coming out the back side going to the motherboard; that's the positive input and for the negative the case of the jack should provide the ground. You should see the same voltage as the output of the AC adapter, about 20 volts between those two points. Wiggle the plug and verify that the power doesn't cut in and out when you do to be sure there's no damage to the solder pads of the power jack.
Let us know what you find and we can go from there.