Sorry but you are going to need to seek the help of a local tech, you getting no signs of life when pressing the power on button means one of two things, the power button or board are bad and/or the charger is bad and not able to charge the battery, if the notebook was working three weeks ago and has not been used since I can`t think of anything else that it could be.
It was working 3 weeks ago but was not charged up.
After looking back at your OP to make sure that nothing had been missed the above now looks a bit off and especially after the troubleshooting steps that have been done.
If the computer was working three weeks ago but it was not charged up it needs to be asked how and why, if the battery was in place but not charging up it suggests that there was already a problem three weeks ago, the alternative being that the battery had previously been removed for some reason and the AC adaptor had to be used, this again suggests that there was already a problem three weeks ago.
It is a used pc
How long have you had hold of this notebook and has it ever worked while it has been in your possession, for all we know the seller may have told you that it worked three weeks ago but has never worked since you have had it.
Is my memory playing tricks on me or do you not repair and sell on computers.