Make sure your FIRST downloader preferences are correct. You might have switched projects or deleted the file or something...
Assuming your .out file builds fine...
Go to Window->Preferences...
Go to FIRST Downloader Preferences and input your team number and navigate to your <Project Name>.out file. It should be somewhere in workspace/<Project Folder>/PPC603/Debug (or something of that sort)...
Either way, look for it in the project folder... Press OK and try again when you get comms...