External hard drive - lost data

[Japamor]

I transferred about 35 Gigs of files onto my external hard drive prior to doing a clean reboot.I then transferred another 3 Gigs. The external drive (40 Gig) is full and I can't get any more on it but when I try to access the 35 Gig transfer it just doesn't show up. I've done it several times before without a hitch. I can access the 3 Gigs but not the original 35 Gigs. Can any one help please? Many thanks.