Why does bash display `>` instead of executing my command?

Why does bash sometimes refuse to accept my orders by simply starting a new line beginning with a greater-than sign instead of executing the command? Every press on enter adds a new line, and the only way to escape this seems to be hitting Ctrl+C.

As an example: a small command that I sometimes need to limit the fan speed of my laptop is not working anymore:

RegenbogenBook:Resources Vincent$ smc -k F0Mx -w $(python -c 'print hex(2800 << 2)[2:]’)
>
>
>

I have the impression that I'm only missing something really obvious, but this kind of basic thing is just not covered in any FAQs or accessible via search...