OLED 0.96″ 128×64 I²C SSD1306
There are many versions of this module. The cheap Chinese module widely available usually has 0x3C I²C address, which is different from the more expensive modules which have better documentation. Using the wrong address will send you on a wild goose chase, checking all sorts of things until you figure out the problem. An easier solution is to use an I²C address scanner.