STM32H745xi enter standby mode after power on 3 seconds

I use STM32H745xi discovery board. When i do some test, i download code that Cortex-m7 will enter standby mode after power on 3 seconds. And i set wrong wakeup pin(wakeup pin 1&not config PA0, it should be wakeu4). After power off and power on, i find that it can't recognize the mcu through ST-LINK though i try RST pin. I dont understand why RST pin not work.

Pls help to save the board,thx.

code:

 MX_GPIO_Init();
HAL_Delay(3000);
HAL_PWR_EnableWakeUpPin(PWR_WAKEUP_PIN1);
while (1)
{
HAL_PWR_EnterSTANDBYMode();
HAL_GPIO_TogglePin(GPIOJ,4);
HAL_Delay(500);
}