In the short term, it is hard to argue that Charlemagne had any great impact much after his death. He did, of course, put together a large empire. However, this empire did not really outlive him at all. It had been held together largely by his own personal characteristics and started to fall apart immediately after he died.
In the long term, some historians have called Charlemagne the "father of Europe." The idea behind this is that Charlemagne managed to combine Christianity, the Roman influence, and German ethnicity in one empire. By doing so, he moved Europe back towards being one culture that was Christian and based on Rome, but which also included the ethnic groups of northern Europe. For the first time since Roman days, much of Europe was united. This, historians argue, created the idea of a European identity. This constitutes a very important way in which Charlemagne can be said to have had a long-term impact.