That's odd.
Exactly how small is it? How big was the original log file? What does file bla.log.gz say?
Normally gzip bla.log will write bla.log.gz and then, if that was successful, delete bla.log. gunzip blo.log.gz will write bla.log and then delete bla.log.gz.
If something goes wrong during the gzip command (say, it ran out of space, exceeded your quota, was killed by a signal, or the computer lost power), then you could easily end up with a partial or corrupted bla.log.gz file -- but then the original bla.log file should still be there.
And if you then run gunzip bla.log.gz, you should get a prompt like:
gzip: bla.log already exists; do you wish to overwrite (y or n)?
It shouldn't have clobbered your original bla.log file unless (a) you told it to (by using gunzip -f or by anwering y to the above prompt), or (b) it successfully generated bla.log.gz.
To answer your question, if bla.log.gz consists of nothing more than a few zero bytes, you're not going to get anything useful out of it.
