Compiling Actionscript 3 in Flex SDK

I am trying to switch from using Flash IDE to Flex SDK. Uptil now, I had been writing actionscript on the timeline. I tried modifying 1 such code to create an external actionscript with a package and class as per the rules and tried to compile this using the SDK. Although an swf file did get generated, it was completely blank. No compliler warnings or errors were generated. What might be going wrong?