croping in Lightroom: when i use a crop different from the original, like 1:3, or an existing one suddenly the photo is gone and i have find it again. The crop is okay. it used to be different: just staying visable...
We can probably sort this out with some more details about the issue.
When you say "suddenly the photo is gone", do you mean that as soon as you apply the crop, the photo disappears from Develop, and you see a blank (gray) Develop window with the words "No photo selected"? If so, then most likely you have a filter applied in the Library Filter bar or are using a smart collection that searches photos by aspect ratio or whether they're cropped.
To troubleshoot that, select a photo to edit, then in the left column of Library in the Catalog panel, select All Photographs. Then make sure Library > Enable Filters is unchecked. Now go into the Develop module with the selected photo. Does it disappear as soon as you apply a crop?
If you mean something else by "suddenly the photo is gone", please provide more details about exactly what you're doing.