Recently I had VP2 mainboard which didn't want to turn on. Completely dead, but once enabled by itself during car transportation. So dead, but can work sometimes.
Davis VP2 mainboard is built on ATMEGA128L microcontroller, but that have you already known.
Microcontroller has built-in RAM + ROM(program memory). To operate it's, you just need nothing more but connect power supply and Crystal Oscillators (XTAL).
First thing to do, is checking power supply if microcontroller has VCC - 3.0 volts, and GND is proper connected. You can use any multimeter to check this parameter.
Second is to check RESET pin, it should be also 3.0 volt. That because if it's 0 volt then microcontroller won't startup. Normal procedure is to connect RESET with GND for short time to proper reset microcontroller, it will set program on the beginning and proper execute it. You can do this manually, but all microcontrollers has special components to reset it properly on startup.
When you examine dead mainboard, you should check RESET. You are looking for anomaly.
If you have power OK, and reset is high (3.0 v). Microcontroller should works, but there is 3rd thing XTAL oscillator. It's a very rare that XTAL can be damaged, but mine was.
That was last thing to check. Fortunately(or not) I have deep fried VP2 mainboard damaged by lightning, and WeatherLink IP to complete set. I have borowed XTAL from this damaged mainboard and place to patient one. A miracle happened. Dead mainboard is not dead anymore.
I was lucky with this one. Don't buy new mainboard before you don't check power supply, reset pin, and maybe XTAL.
XTAL has also 2x capactiros 22pF. If one of those are damaged, XTAL won't work, so microcontroller won't work as well.