Web applications on uncompromised computers are vulnerable to XSS,CRSF,sql injection attacks and cookie stealing in unsecure wifi environments.
To prevent those security
Am I now invulnerable to web-based hacking attempts?
Because, no matter how good you are, everyone makes mistakes, the answer is no. You almost certainly forgot to sanitize some input, or use some anti-forgery token. If you haven't now, you or another developer will as your application grows larger.
This is one of the reason we use frameworks - MVC, for example, will automatically generate anti-CSRF tokens, while LINQ-to-SQL (as you mentioned) will sanitize input for the database. So, if you are not already using a framework which makes anti-XSS and anti-CSRF measures the default, you should begin now.
Of course, these will protect you against these specific threats, but it's never possible to be secure against all threats. For instance, if you have an insecure SQL-connection password, it's possible that someone will brute-force your DB password and gain access. If you don't keep your versions of .Net/SQL-Server/everything up to date, you could be the victim of online worm (and even if you do, it's still possible to be zero-dayed).
There are even problems you can't solve in software: A script kiddie could DDOS your site. Your server-company could go bankrupt. A shady competitor could simply take a hedge-clippers to your internet line. Your warehouse could burn down. A developer could sell the source-code to a company in Russia.
The point is, again, you can't ever be secure against everything - you can only be secure against specific threats.