Cannot flash programming more for a LPC2148 board

I am writing some codes related to watchdog timer. After I flashed the binary generated by the following code, it is found that there is no way for me to do flash programming anymore using Keil Ulink2. I always get errors of "Cannot enter Debug moder" in the build output and "Cannot Load Flash Programming Algorithm" will be shown in a popup error message box. Can anyone help? Thanks.

unsigned int delayInMs = 60000;
void feedWatchDog() {
    WDFEED=0xAA;
    WDFEED = 0x55;
}

void delay(int val) {
    int cnt;
    val = (val >> 2) << 16 <<7;
    for (cnt = 0; cnt < val; cnt++) {
        feedWatchDog();
    };       /* Delay */

}

main() {
    int n;
    int delayVal;

    PINSEL1 = 0;
    IODIR0 = 0x00FF000;


    if (WDMOD & WDTOF != 0) {
        delayVal = 100;
    } else {
        delayVal = 10;
    }

    WDTC = delayInMs * FOSC * PLL_MUL / (VPBDIV_val * 1000);
    WDMOD = 3;           //watchdog reset mode
    feedWatchDog();

    while (1) {            /* Loop forever */
        for (n=0x00001000; n <= 0x00080000; n <<= 1) {
        /* Blink LED 0, 1, 2, 3, 4, 5, 6, 7 */
            IOSET0 = n;           /* Turn on LED */
       delay(delayVal);           /* Delay */
       IOCLR0 = 0x00FF000;        /* Turn off LEDs */
    }
  }
}