You say you connect the thing to D8 but again that's impossible on the ESP-01. D8 is a NodeMCU/WeMOS designation of the pins, in an attempt to be more Arduino Uno like. You're confusing yourself and the forum readers by using those indications, which are not even marked on the ESP-01 as such.
On an ESP-01 you have four pins available: Tx (GPIO 1), Rx (GPIO 3), GPIO 0 and GPIO 2.
Tx has a blue LED connected to it; that will light up when you use it as input and bring the pin high.
Rx has nothing special if you do not use the Serial interface.
GPIO 0 is used in the boot sequence, and must be pulled HIGH for the ESP to boot.
GPIO 2 is used in the boot sequence, and is pulled high internally to boot; pulling it LOW externally (e.g. with your voltage divider) will stop it from booting.
So using either GPIO 0 or GPIO 2 for your voltage detector will stop your device from booting.
Please show a schematic of how you have wired things. No Fritzing wiring diagrams. No "I've used a Nano in the image but it's an ESP" laziness, that's useless and only adds confusion as you've seen already. Just use pen and paper if you don't have real schematic software such as KiCAD (which is free!) at hand.