Had the same thing on my netbook before. Happened after a clean install (i.e. not the OEM preinstalled OS).
Ended up being a driver problem/incompatibility preventing the netbook from going to sleep. It was trying to and had already turned the screen off, just to immediately come back to life. A very outdated LAN (wired ethernet) driver was the culprit in my case.
I'd suggest updating the main drivers, i.e. chipset, graphics, LAN, WLAN/WiFi and sound.
I did it in a step-by-step fashion. Like: update one driver, reboot and try if sleep mode works. If it doesn't, it comes back to life instead of going to sleep and you can install the next driver
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