Why the screen resets every second?

Question

I am trying to complete my project at university but I cannot find why my screen resets after a second. The project supposed to be stopwatch, showing minutes and seconds. I need to have 3 buttons: 1. startstop 2.set minutes 3. set seconds, but it should works.. I have no idea why it set "0000" second after changing it...

Project is wrote to ATMEGA32 processor

#include <avr/io.h>
#include <avr/interrupt.h>
#define F_CPU 16000000
#include <util/delay.h>
#define segment_DDR DDRA    //porty dotyczace wyswietlacza
#define segment PORTA
#define digit_DDR DDRB
#define digit PORTB
#define keys_DDR DDRC  //porty dotyczace klawiszy
#define keys_PULLUP PORTC
#define keys_PIN PINC
#define key1 0
#define key2 1
#define key3 2

volatile uint8_t button1=0, button2=0, button3=0, startstop=0; //deklaracja zmiennych

uint8_t seg7[] = {0xc0, 0xf9, 0xa4, 0xb0, 0x99, 0x92, 0x82, 0xd8, 0x80, 0x90}; //tablice
volatile uint8_t data[] = {0, 0, 0, 0};

ISR(TIMER0_OVF_vect) //timer0
{
    static uint8_t i=0, dot=0, mig=0;  //odswiezanie wyswietlacza
    digit |= (1<<i);
    if(i++==3) i=0;
    segment = seg7[data[i]];
    
    if((i==3) && (startstop==1) && (dot++==50))
    {
        dot=0;
        mig ^= 1;
    }
    if (i==3)
    {
        segment = seg7[data[i]] & 0x7f;
        segment |= (mig<<7);
    }
    digit &= ~(1<<i);

    //button startstop
    if (!(keys_PIN & (1<<key1)))
    {
        switch(button1)
        {
            case 0:
            button1=1;
            break;
            case 1:
            button1=2;
            break;
        }
    }
    else
    {
        switch(button1)
        {
            case 3:
            button1=4;
            break;
            case 4:
            button1=0;
            break;
        }
    }
    //przycisk_m
    if (!(keys_PIN & (1<<key2)))
    {
        switch(button2)
        {
            case 0:
            button2=1;
            break;
            case 1:
            button2=2;
            break;
        }
    }
    else
    {
        switch(button2)
        {
            case 3:
            button2=4;
            break;
            case 4:
            button2=0;
            break;
        }
    }
    //przycisk_h
    if (!(keys_PIN & (1<<key3)))
    {
        switch(button3)
        {
            case 0:
            button3=1;
            break;
            case 1:
            button3=2;
            break;
        }
    }
    else
    {
        switch(button3)
        {
            case 3:
            button3=4;
            break;
            case 4:
            button3=0;
            break;
        }
    }


}


ISR(TIMER1_OVF_vect) //timer1
{

}

int main(void)
{
    keys_PULLUP |= (1<<key1)|(1<<key2)|(1<<key3); //pullup
    keys_DDR &= ~((1<<klawisz1)|(1<<klawisz2)|(1<<klawisz3)); 
    segment_DDR = 0xff;
    digit_DDR |= 0x0ff;
    digit |= 0x0ff;
    segment = 0xff;
    TCCR0 = (1<<CS01);
    TCCR1B = (1<<CS12)|(1<<WGM12);
    TIMSK = (1<<TOIE0)|(1<<OCIE1A);
    OCR1A = 62500;
    sei();
    while (1)
    {
        if (button1==2)
        {
            startstop = !startstop;
            button1 = 3;
        }

        if(startstop!=1 && button2==2)
        {
            if (data[3]++==9)
            {
                data[3]=0;
                if (data[2]++==6)
                {
                    data[2]=0;
                }
            }
            button2=3;
        }


        if(startstop!=1 && button3==2)
        {
            if (data[1]++==9)
            {
                data[1]=0;
                if (data[0]++==6)
                {
                    data[0]=0;
                }
            }
            button3=3;
        }
        
        while(startstop==1)
        {
            if (data[3]++==9)
            {
                data[3]=0;
                if (data[2]++==6)
                {
                    data[2]=0;
                    if (data[1]++==9)
                    {
                        data[1]=0;
                        if (data[0]++==6)
                        {
                            data[0]=0;
                        }
                    }
                }
            }
            _delay_ms(1000);
        }
    }
}