I have just replaced the hard drive in my PC and now Lightroom 5 does not see my photos. I can't see the photos in their folders on my pc, and I cant see the collections which I made.
Did you transfer and open the catalog from your old hard drive? The basic principles are the same as: http://www.lightroomqueen.com/how-move-lightroom-to-new-computer/
Not necessarily. Any decent computer consultant will have transferred the files to the new hard drive (or at least have done backups), so do a search for *.lrcat files. That would be your catalog, which would contain all of the information about your photos. Then you just need to find your original photos on the hard drive, just I case they've moved.