"Your original files are safe. Whether the tarball you made is readable is a whole different question -- whether you can a) gunzip it and b) untar it are two different potential points of failure. I don't think they are recoverable, but I'm really not sure.
I wouldn't risk the chance of a corrupted backup file -- delete it and start the job again.
Tip: run screen first, so if your terminal gets disconnected for some reason, the tar will continue."