I accidentally left my Raspberry Pi on overnight. When I discovered that the next day I also found that I couldn't get it to respond. I've tried cycling power but never gotten it to respond since then. If I recall correctly, the red led is on but the green led which used to flash on and off no longer comes.
I'd like to be able to leave it on for extended periods for some of the things I'd like to do. Is this a heat issue? I did apply the little heat sink to the CPU chip on the board but don't know what to do with the other small piece that was supplied with the kit that had the raspberry symbol on it. Does that matter?
There should be no problems when leaving a Raspberry Pi running for long periods. I have a Pi 3 in my network that has been running for 11 months. Please tell us a little more about your setup.
Have you got anything wired directly into it, or one of the PiHats for example? If so disconnect all those and try again
Otherwise I would burn another SD card and see if that fixes it.
If not then it may be broken.