i go "cast magic" in the game screen and i click the arrow button for more spells, and more than 20 of my spells near the end are browned out, and i can't use them

HELp!

And I have an assasin How do you use backstabb????

Read the manual, it's great for things like this... I'll make it easy for you, I suppose.
You're probably using a mage/cleric, and wearing armour. Cleric spells will work, but mage spells cannot be cast when you're wearing armour.
Backstabbing is hitting an enemy in the back while you're invisible either via the stealth ability or an invisibility potion.

Maybe your intelligence has dropped since you learnt the spell, and your current score is too low to cast it eg you have been drained or you drank an intelligence boosting potion to learn the spell in the first place. Other possibility is you have been level drained, check your stats screen.