At a guess - and without your code that's all it can be - you are testing the key press to see if it is within the range of "a" to "z". Something like this:
private void myTextBox_KeyPress(object sender, KeyPressEventArgs e)
{
e.Handled = !(e.KeyChar >= 'a' && e.KeyChar <= 'z');
}
All you need to do is expand that to cope with upper case characters as well:
private void myTextBox_KeyPress(object sender, KeyPressEventArgs e)
{
e.Handled = !((e.KeyChar >= 'a' && e.KeyChar <= 'z') || (e.KeyChar >= 'A' && e.KeyChar <= 'Z'));
}
Or just:
private void myTextBox_KeyPress(object sender, KeyPressEventArgs e)
{
e.Handled = !Char.IsLetter(e.KeyChar);
}