Hi Remco_H - Netherlands,
Which version of Lightroom and which Camera are you using?
Could you post a screenshot of Lightroom import Window?
I have lightroom 5 and sony A77 M2. Finally i did solve it myself investigating other posts on internet about same topic. I did click on the destination area part of the " Copy to Area" (Rightside of screen) in Import modus and the pictures showed up again. Still do not know why the pictures did disappear and hope it was just something I did wrong somewhere. Before I had multiple tasks (moving files) in background maybe cause some pressure on system/software.
Thanks for the reaction anyway
Thanks for the update
Good to know you've fixed it.