Great job trying to troubleshoot! However, this is a result of the flash engine reaching its limits. For whatever reason, while you were editing your Prezi, you reduced the font size down to its limits and now they need to be enlarged again. Please shift select these words and increase their size.
First zoom in/out as much as appropriate in order to be able to select the object(s). From here left click and hold from the top left corner of the image to the bottom right and release. From here the zebra should appear and you should be able to modify the objects now.
Good luck and happy zooming!