One possible reason is that you might have used the app to clear the device memory/cache/history to make the device perform better, but this could also clear the data on PicCollage as well, and this is why the previous collages and the sticker thumbnails are missing.
We have been working on it to prevent the same problem happening again in the future. The latest update (4.1.4+) should not have this problem if you create a new collage, so, please try to download the latest version, and then create new collages and it should be saved this time.
Furthermore, you can also save the collages to your Gallery or share to PicCollage in the future, so those collages will be stored in device gallery, or under your profile and you can review them every time you are logged in.
However, for the missing sticker thumbnails, you will need to reinstall the app to make it show again. The collages will be removed once the app is reinstalled, so please first share the collage to your collage, or save it to your gallery before you reinstall the app.
To save to gallery, please tap on the Share icon on the lower right corner of the collage canvas, and select Save to Gallery.
In the meantime, please make sure that when you are cleaning device data/cache using another app, it doesn't clear the data/cache from PicCollage.