Lenovo won’t turn on

I was working on my computer, battery was charged, and computer was unplugged. My computer turned off out of nowhere. I tried to turn it back on and the red light flashes but nothing happens. Tried charging it and nothing. Took the back off, unhooked the battery and put it back together, still nothing. Can’t work without computer and would love to not have to buy a new one. Got the computer approximately 4 years ago. Help!