You only have the one save available? Usually the game keeps 2-3 autosaves along the way (unless you have that turned off... if you do, whether you solve this issue or not, you should consider turning that option on).
As for getting it to load, sometimes the Fade cutscenes can have issues.
Few questions, what version of the game are you running? Second how long have you waited after you beat the Sloth Demon? It's supposed to pop up immediately, but sometimes it does not. Also, do you have any third party mods running?
If you don't have or are pretty sure no mods are causing the problem, one way I know you can get it to work is to use the console to hopefully force load the cutscene. To do so you first need to start the game using the console option.
Assuming you don't know how to do this, first you need to modify the game's shortcut and add -enabledeveloperconsole at the end of the target area in the shortcut properties. So for me it would say
"C:\Users\Public\Games\EA\Dragon Age\DAOriginsLauncher.exe" -enabledeveloperconsole
Then run that shortcut and press the ` (far left corner above the tab, near ESC) key. Now, thing is, the game does not include the font that allows you to see what you type. You can find a mod at http://dragonagenexus.com/downloads/file.php?id=1110 that adds the font.
Then, once you have the font working and the console working, type runscript zz_cir_debug which should allow you to "cheat" so to speak by modifying plot flags and teleport around the fade. Hopefully you can use this to set it all as complete and then teleport back to Niall for the cutscene. If the cutscene doesn't show, then at the very least you should be able to get out of the fade to continue the game.
