First, turn off background rendering, and never turn it back on.
Second, select the Library in the Browser, from the File menu “Delete All Generated Media” and from there delete all render files. (Or just purchase Final Cut Library Manager.)
Never trash anything inside the Library bundle manually (again, get Final Cut Library Manager), very bad idea, unless you really know what you’re doing.
Do you still have the original media? Where is it? When you go to the Viewer menu, are you set to use optimized/original or proxy media? Make sure you set to use optimized/original media.
What do you mean by “transcode it”? Do you mean export it to a file? Not sure what you mean there, or what is dimmed.