Photos frozen "preparing library" from iPhotos

My son updated the software on my computer to OS X Yosemite. Now I can't use iPhotos and instead have this new Photos program. I followed the instructions for set up and selected my iPhotos library and it got stuck "preparing library" at 24%. I got an error message after an hour or so saying there was an error. I tried again, seemed to go ok until it hit 24% and again it's been stuck there for nearly an hour. I have over a decade of photos on iPhoto and I can't afford to lose any of them. I can see under my Pictures in Finder that there is a Photos folder and it appears to contain almost all (if not all) of my photos from iPhoto...But launching and using Photos just isn't happening. Any help would be much appreciated! I don't really know what I'm doing and my daughter is helping (a Windows user...). Thanks.