Has anyone had a problem with a paladin being unable to cast spells? He is a Paladin of Helm and is of sufficient level to cast spells. The spell casting icon on the quickbar doesn't show up.

You might need to add it manually (right-click to do that), or if you mean that things are greyed out you probably have too low wisdom.

I'm have Paladin of Helm 10 level and no casting problem it is...

Yes it's maybe you paladin have low wisdom (6 or lower)

Or your paladin is "fallen", you've done too many bad deeds and your rep is too low then the paladin loses his/hers powers.

Yes. Bluerose is right. You need to add the icon manually or your character has very low wisdom. :p