Precision M3800 Not Turning On

I have a client that has a Precision M3800 from ~2015 (warranty expired in 2017). The computer was dropped off to me to do some software updates/upgrades, and worked fine yesterday. I powered it on without the power adapter, and it had a decent battery health for its age. Today, it would not power on at all. Pressing the power switch makes the light (on the power button) turn on for about 5 seconds, with some extremely faint fan-like sound if you put your ear right up to the computer. After 5 seconds, the light goes dim again. Nothing appears on screen at all, so I can't go into the BIOS or do anything. I've tried unplugging everything, holding down the power button, etc., but nothing changes. The user said that every 2-3 months or so it has trouble turning on, but after leaving it plugged in for 2-3 hours, it should turn on. This has not worked for me. The only thing I can think of right now is opening the cover and taking a look inside, but before that, I wanted to see if anyone has any suggestions.