If you are using the client side validatiosn, then things should work fine unless some smart guy is trying to disable javascript and send blank data.
Better to use the requiredfieldvalidator.
http://www.devmanuals.com/tutorials/ms/aspdotnet/requiredfieldvalidator.html[
^]
If you want to be more safe then check the textbox null or empty in server side code.
if (!string.IsNullOrEmpty(textbox.text))
{
}
I will also like to suggest you to use parameterize query. This will help you from SQL injection.
Check^