Lets say Constaninople fell in the second Arab siege in 718. Can you find one European power that would actually be able to stop the Arabs, Keep in mind that the Slavs and the Bulgars as pagan would be highly suspectible to convert to Islam giving more life to the Arab conquests.

PS. Let's not even think about Constantinople falling in the first Arab siege in 674. There would be absolutely nothing that any European state could do to prevent Islam from reaching Cadiz...the long way around!