We need more information to understand what is happening. If it was happening to everyone, there would be a serious problem with Lightroom.
Which version of Lightroom? Which operating system?
What exactly do you mean by losing the files? You import them into the catalogue. They import OK. Next time you launch Lightroom, they are not in the catalogue?
Thanks for replying, I am using Lightroom CC 2015 with Windows. The photos imported fine, and I was working on them yesterday. But now all my folders are gone, I can find smart previews for all the photos, and by clicking on the exclamation point I can find them in my files. I did import them using a different computer (I study, and I imported yesterday using a Mac at uni, however I never had this problem) I went to launch today and none of my folders from this year are there
I did import them using a different computer
How did you get them from the Lightroom on Mac to Lightroom on Windows?
I have my lightroom catalogue on my portable hard drive
Each time you reconnect the external drive to the Windows computer is it identified with the same letter? Windows will attach a letter to a drive by using the first available one, in alphabetic order. For instance, a drive may have been identified as "F" yesterday, but today when you hook it up there is already a card reader being identified as "F", so the drive will be listed as "G". This changes the path to all the contents of the drive and confuses LR. You can avoid this happening by using the Device Manager to assign a letter to the drive which is toward the end of the alphabet and therefore will always be available. My external drives are W, X, Y and Z and are always recognized as such on any Windows system.