Question Dell Inspiron 15 N5040 not starting up

Sep 3, 2022
My Dell Inspiron N5040 laptop will not turn on. It is coming up to 11 years old (I know, it’s old) but worked fine if a bit slow before recently when this problem arose. It is not the CMOS battery, and I only just took it apart and cleaned it thoroughly a couple of years ago, and reapplied the thermal cooling paste, so I know it’s not that. It has not had a battery in it for at least 5 years as it died and was causing an error message and beeping problems, so I took it out and just had it connected to the charger at all times, which worked but was a nuisance when it would fall out and the laptop would switch off immediately.
Now, recently, I’ve not been using the laptop as much since I got a new job and they supplied me with a Mac to use as I work from home some days, but it’s purely for work use. I probably didn’t use the Dell laptop for about two months and it wasn’t plugged in during this time, and stored vertically on its front edge (not sure if that makes a difference) . When I tried to plug it in and turn it on i pressed the power button as normal and for about a second it sounded like it was powering up and the fan started but almost immediately it made a power down noise and wouldn’t come on. The power light came on briefly and then about 5 seconds later it tried to power on again automatically, but did the same thing. If I left it it would do this for a while. I decided maybe because it hadn’t been plugged in for a while and needed charging or something, so I left it with the charger in and closed it. Then last week I tried again but it still does the same thing. I’ve took it apart again , dusted it and replaced the CMOS battery just in case, and checked all the wires were connected to their slots, pressed the power button while the casing was off and could see the fan start up but after only about one and a half seconds it immediately slows down and stops as the laptop makes the power down noise.
Not sure what it could be. I’ve tried another Dell charger that fits the laptop just to see if the charger was the problem but neither of them make the laptop work, and both of the chargers have the green light on once plugged in to the socket so I know they are on.
I’m wondering if it could be a problem with the motherboard ? But unsure what that may be. Or if it would be worth getting a replacement battery and seeing if that helps in some way if it’s a power supply problem?
Any help would be appreciated.