I used dragonrend on Alduin at the Throat of the World, and preceded to attack him; I am unable to hurt him at all.

Is there a fix for this?

You have to reload a save and wait till he is done with his entire speech.

That's the only way I know how to fix it.

This is caused by attacking Alduin before the talking is over.

After viewing the cutscene and engaging in battle with Alduin, it is possible that if you use 'Dragonrend' before he has finished talking Alduin will not take any damage whatsoever and/or to become stuck. Also applies when a follower attacks Alduin before the conversation ends (verified with Serana).

• Wait until Paarthurnax tells you to use 'Dragonrend' before you use it.
• A known method of fixing this is by reloading an earlier save from before using the Elder Scroll in the Time wound. Also, leaving the location and fast traveling back to it fixes this. And if you die as defeating Alduin it is possible that you have to repeat this.
• Another method is to travel to High Hrothgar, enter and exit the building, and run back up to the mountain to resume the fight.
• Don't open any menus until Paarthurnuax says "use dragonrend if you know it!"

Thanks, I'll try it now.

It worked, thanks.