This problem could originate from:
Your import file is too large and the script is broken by the server because of the timeout. In that case you can reduce the number of lines to handle in your CSV and use the auto-import tool which will relaunch the import process each 20 lines for example. Please refer to this article for more information.
If the import stops every time on the same line, the problem could be that one of the image file has a wrong file format and we cannot create thumbnails. In that case, you should check the images to ensure the file format is correct before relaunching the import process.