I can try and help if you'd like. I'm also new to Flash Builder/Flex 4 but FB is eclipse based and I have almost 10 years experience getting my mind blown by eclipse.
First off, do you have automatic build enabled for the workspace/projects? If not, try enabling this.
Now, how complex/large is your workspace? how many projects in it? Large library references?
One thing I used to do in Eclipse is delete the '.metadata' folder in the workspace. This basically blows away all of FB/Eclipse configuration settings for your workspace allowing you to start from a clean slate but with the inconvenience of rebuilding all your workspace settings. Workspace/Project corruption seems to be a common "feature" of eclipse and hence I would imagine that this would also happen in Flash Builder.
Another thing to do is create a new shortcut to the Flashbuilder.exe file and then modify it and put a -clean at the end forcing FB to <ahem> clean itself.
Anyway, lets see if you can get to building and rebuilding your projects/workspace and then take a look at debugging.
Thanks embeddedsoft. I did have automatic build enabled. My workspace was very small, 5 projects with 100 lines of code each max. I've messed around with those profiles/metadata folders and done some more of those destructive cleans.
Nothing helped. Then I started a new project just to see if it would run, and it did. So I started copying over line by line of my current project slowly reconstructing it and behold.. the fully copied project worked.
So altho I have no idea what caused it or how to solve it, if anyone has this same problem: Start over and slowly bring in your old code to the new project. Then delete your old projects!