Try copying and pasting the following command into Terminal (/Applications/Utilities/Terminal):
Code:
defaults write -g ApplePressAndHoldEnabled -bool true
This sets the setting back to the default value (true) and should work if the cause was something setting it to a different value. You'll need to relaunch whatever application you're using for typing after you do this.
If you're typing those characters a lot it might also be faster to learn the keyboard shortcuts for them. For the accented vowels, type option-e followed by the vowel (eg opt-e followed by u gives ú), and for ñ type option-n followed by n.