Usually 'lost' is a euphemism for "We stopped paying the developer and now he wont give us the source code."
That being said, I own a copy of Burak's ActionScript Viewer, and it works pretty well. A simple google search will find you many other SWF decompilers.
Get the Sothink SWF decompiler. Not free, but worth it. Recently used it to decompile an SWF that I had lost the fla for, and I could completely round-trip swf-fla and back! link text
I've had good luck with the SWF::File library on CPAN, and particularly the dumpswf.plx tool that comes with that distribution. It generates Perl code that, when run, regenerates your SWF.
I've used Sothink SWF decompiler a couple of times, the only problem is that as project gets more complex, the output of decompiler gets harder to compile back again. But it ensures that you can get your .as files most of the time, compilable fla is a question.