The flag should be typed with double preceding dashes and a space.
Like this: --restore-last-session
Make sure you get this right.
Renaming Last Session to Current Session should work too.
If none of the above methods work out for you then try replacing the entire User Data folder. Go to AppData\Local\Google\Chrome and make a copy of the User Data folder, for instance by copying it to Desktop. This is just a safety measure so that you can revert back to it, should you need to. Then go ahead and delete it. Make sure Chrome is not running when you do this. Now go ahead and copy your old User Data folder in its place.
The next time you start Chrome it will say something along the lines "looks like Chrome has crashed, would you like to restore" and a restore button will appear on top. Click the restore button.
This method worked out well for me when I was trying to restore an old Chrome session to retrieve my tabs.