I'm a templar, so I may be able to help you out on the changes to our classes since you left, as much as they cross over. Also some general tips on playing our classes now.
It's probably best to wipe your spell bars, open up the spellbook and start from scratch.
Big change number 1: Your single or group reactive heals will not stack with lower level versions any more, they will stack with each other, and with the emergency reactives, if you Inqies get those.
Wisdom effects your power pool and resists. No stat effects healing power.
Most of the new spells you got are for levels 1 to 20, becasue we no loger get generic priest or cleric spells at those levels. Some of the new ones will be changed or new since Live Update 13.
New spells all come at the beginning of a level now, no more new spells at level x.5.
Most buffs aren't on timers any more, they last until you cancel them.
Spells do not increase in power as you level any more, the only way to increase them is to buy, make or find upgrades. Your level 14 Adept I spell will no longer be better than your Apprentice 1 level 28 spell. As before you still start with Apprentice 1, and you can upgrade to Apprentice IV, Adept III or Master I. You have the option to upgrade one spell at level x4 to Master II. I've found that upgrading our heal spells makes a MUCH bigger difference than other classes and spell types tend to see. Some spells do upgrade with level (just to be confusing) - they're the ones with no app, adept, master grade level.
We no longer get group cures (poison, arcane, elemental, etc.) as options when leveling. We get every single target version and one group version. The single target ones automatically upgrade with level the group one has to be upgraded. I got the group arcane cure, you might get another.
Your nukes are weaker, but MoB's are also weaker in terms of hit points. Intelligence effects nuke power. Debuffs increase nuke power slightly but really help to reduce resists.
Your debuffs (this applies to you more than me) will make a difference in the speed of taking down MoB's, but not as much as before and only if the fights last long enough to cast them in the first place.
Try to get your groups to fight tougher MoB's if you feel like you're not doing enough. Clerics shine vs strong, melee oriented enemies. Slow, big damage hitters are tougher for us to deal with because of the nature of reactive heals and because our instant heals are on longer cast and recast timers than other priests.
You can learn what most of your spells do by soloing. It's good practice. Try to fight even con (white) non-heroics (no arrows) and groups of non-heroics (down arrows) to start. If you try to solo orange con MoB's they'll be very tough, they hit hard and resist your spells a lot, you will not be able to resist them. Red cons are just suicide for anyone now, even in groups.
If you're getting beat on, you'll find you're getting a lot more interrupts than you did before. We lost parry and they changed heavy armor properties so our avoidance dropped drastically. Some people (like me) say this is unbalanced and it should be changed - plate wearers getting hit a lot is fine, but when hits interrupt casters as much as they do it seems unfair. It seems like they changed things because of plate tanks (it used to be possible to make them nearly invulnerable) without compensating for plate casters. If you're soloing, getting off a heroic opportunity will help you a lot with interrupts because it adds 25 to your focus for 7 minutes, which is equivalent to adding 5 levels of focus ability. (I always spam HO's when I solo) Better armor helps a lot too. I wear rare crafted and I find green and blue enemies can barely touch my health, and they miss a lot.