Super new to this.
Attached the Adafruit 16-Channel PWM / Servo HAT for Raspberry Pi to my Pi 3. All solder connections look good. Booted up the Pi, ran through the tutorial. After tutorial, the hat was detected on the proper channels (0x40 and 0x70, shown from sudo i2cdetect -y 1). Thought I was good, so went to the next step and downloaded the library and ran the servo test. Terminal reported something about no connection (rebooted many times, don't remember the exact message) (Yes, there is a servo connected, on the first set of pins).
So, went back and followed the instructions again, and performed the manual checks recommended. Everything seemed good. Rebooted the Pi3. Now i2cdetect can't find the hat.
uname -a returns that I am running raspberrypi 4.4.50-v7+ #970 SMP
tl;dr, i2cdetect showed the hat on the proper channels, now it's gone.