[mathil]

And did that solve your problem? You should also check if the memory is full, and in general make sure that you are not loading files that are too big on the device. E-readers (all of them, not just the Kobo's) have slow processors and have less RAM than even the cheapest smartphone you could buy nowadays, they are not equipped for big PDFs or CBR/CBZ files. I would say not to load files bigger than 100mb if you don't want to experience your device slowing down excessively or freezing.

IIRC, the collections themselves are saved to the cloud, but if you didn't buy your books from Kobo, you'll have to re-add all your sideloaded books to their own collections. What most people on mobileread do is using Calibre to create them automatically based on tags (etc. series, the author's name etc.). You can find out how by checking the Mobileread wiki's article 'Kobo Shelves and Collections'.