No problem, Nixwinter. We'll do this step by step. 1) Go back to the ePub version before you started editing it (I hope you have made a backup). 2) Open the ZIP file and open the OPF file. 3) Since you have added the cover picture before, most of the meta data are already there. The only thing you should add is
Code:
<meta name="cover" content="cover-image" />
in the metadata section.
4) Now you have to change the word "cover-image" to the name Sigil gave to your cover image. This can be found in the manifest section of the same file. It is probably something like img0001.jpg (if you made your ePub with Sigil) and the name is preceded by id=
5) Save the file and zip all the files again as Valloric said before.
That's it. If it still doesn't work, post your entire OPF file so that we can have a look!