How Can I Find indices of an element in 2D array??

Have you tried simple loops, with right sizes?

int found_i = -1;
int found_j = -1;

for(int i = 0; i < 100 && found_i < 0; ++i)
{
    for(int j = 0; j < 200; ++j)
    {
        if( MyArray[i,j] == (object)sender) // (or maybe 'object.ReferenceEqual')
        {
            found_i = i;
            found_j = j;
            break;
        }
    }
}