It’s possible that there is a short-circuit in the Mayfly, but this sounds more like a problem with your code or sensors.
When you’re posting code, please use the “Add Code Snippet” button. It adds coloring and formatting which makes the code much easier to read.
When you’re troubleshooting the boards, make sure to check the position of all of your switches and jumpers. If the small switch at the top of the board is set to “4-12V” instead of LiPo, the board will not be powered by the computer and thus will not respond when plugged into the computer. Also check all the settings of the jumpers near your Grove ports. Most of then can be changed between 3.3V switched and 5V switched power and the I2C can be changed between 3.3V switched and 3.3V continuous power. Some I2C devices have internal pull-up resistors which can cause the I2C bus to go low if the power is switched off. If the I2C bus is held low for any reason, the Arduino core I2C (Wire) library will crash and the board will completely stop responding. Both the RTC and the auxiliary ADC use I2C to communicate, so the crash might happen when you’re talking to one of them instead of when you’re talking to a sensor.
I am not familiar with the particular OLED screen you are using, but I wouldn’t recommend any screen for field deployments because they require a fair amount of power. If you do not have one attached, you should probably remove the code for it to help make troubleshooting easier.