Kaika,
Like other people have said, try and recall something that wasn't too traumatic (or even make something up if you REALLY can't think of anything) your teacher is just trying to get you to work on descriptions and using something that you've experienced is a great way to do that because you know how you felt. I do think it's distasteful getting you to write about something that horrified you, but you generally experience more when something scares and upsets you than, say makes you happy.
Have you ever had a pet go missing even for a short while? Or, maybe something really important and sentimental to you go missing and you panicked? Or watched a film that was really gory? Just some suggestions, it doesn't have to be anything really horrific, as long as you're describing emotions and reactions they're not going to mind what it is.

Best of luck,
Molly