You should do something like this:
int tempVal = 0;
if (int.TryParse(Microsoft.VisualBasic.Interaction.InputBox("Please enter value for Day " + (day + 1) + " in week " + (week + 1) + "."), out tempVal)
{
toys[day, week] = tempVal;
}
else
{
MessageBox.Show("Invalid entry");
day--;
continue;
}
int.TryParse()
will parse the data into an integer, and it adds a check for valid data.