There is the siege mission where you ride the water stag. I rode it across the suspension bridge during the mission and the cutscene plays and you were dismounted.
After the mission when I popped back up in normal world I can no longer summon mount. Either via the hotkey or via the Alt-V Mount UI and clicking the Ride button.
Anyone know of a fix? I really don’t want to log off and face the queue.
Hi @Karamethien
Hope you doing great!
I know you don’t want to log out of the game cause of the queue we are having right now but in this case you have to try to check the integrity of the game files and after doing that open the game again to verify if you can use the mount again without problem.
You can do it by following these steps:
If you are still with the same issue after you try the troubleshooting, I recommend contacting our live support to help you create a ticket to the devs team and help you further: Contact Us | Amazon Games
Hope this information help you! 
Hi Kagami,
Thanks for the advice. I ended up braving the queue. Once I restarted the game and once I managed to login it is back to normal.
Guess it is just one of those momentary hiccups.
Hi again @Karamethien
Thank you to let me know that restarting the game resolved the issue.
Hope you can continue you adventure in Lost Ark and enjoying it!