<p>What is a security sandbox violation, and how do I override it? I'm making a project in FlashDevelop, and today, it will not load external files, even though it had been previously. It gives the following output:</p>

<blockquote>
  <p>************* Security Sandbox Violation *********   Connection to
  file:///C|/Documents%20and%20Settings/Kevin.PC885314341208/Desktop/Import%20.txt/StockData/1.txt
  halted - not permitted from
  file:///C|/Documents%20and%20Settings/Kevin.PC885314341208/Desktop/Import%20.txt/import.swf</p>
</blockquote>

<p>I hope this isn't a serious problem.</p>

<p>Thanks for any help.</p>

You're loading a local file, something that's not permitted. However, as you're developing this you can of course allow this.
Add your project folder to the trusted locations:
http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager04.html
Edit: I might add that if you run it with networking disabled it will work (this is for development purposes). This happens for example when you run it from the Flash IDE, which would explain why it may have worked before?