FOR THE TOP LANE
The people who got nerfed or fell off since you left
Riven for the first time in ages is legit trash tier, the only people who can play her are legit good Riven players, the new masteries and how she was nerfed left her in tears </3
Gnar while a highly contested pick in season five, is now a mediocore champion, he's not great, but hes easy to counter pick, and does not do well in the meta of marksman. In team fights you either pick a tank or a carry for top lane, Gnar does neither and serves as a in between role, which is currently bad in the modern meta for top laners. In top lane it's pick a carry or a tank, no inbetween. He is bad for a number of reasons in the current meta, but I will move on for now.
Vlad Suffers for a similar reason gnar suffers, is neither a tank nor a carry but serves his own unique role that is no longer needed for the current meta. Vlad in general has always been a niche pick, but suffers even more so in the modern meta
Swain, stupidly got nerfed to oblivion for riot was afraid he was going to become op, is legit trash tier unplayable right now unless you miraculously get a early lead
While not destroyed, his lane presence was heavily nerfed with his ult being reworked into a team fighting tool instead of a trading tool.
The people who are strong currently
Fiora rework made her into a solid carry for your team, offers lots of out play in a multitude of match ups, while haveing a team fight usefull ultimate where once the target dies you basically get a janna ult circle that heals your team, great split push, great team fight.
Malphite, debatebly the best ultimate in the game
While not favored by pro players, has super strong carry potential in solo que and low elo from the fact late game you get this late game tank who can't be killed who also does massive amounts of damage with a execution that can nuke anyone.
Natulist, brings huge utility for your team, auto wins lane
The king of top lane is back, everything you hated about him in season four? Yeah it's back.
Things that were left unchanged
Renekten
Still good
Irellia is still Irelia
Things that were reworked
Poppy
Amazeing rework, is a legit good champion now for top lane and jungle
Shen
Is now a duelist instead of a poke/harrass, still good in my opinion, what makes shen is his ult and taunt, and he still has both
Sion
In my opinion is the more skill demanding version with nautlist who uses stuns instead of slows.
Gangplank
Was legit amazing in his rework, the best champion in the game debatebly, but was recently majorly nerfed, it is currently unkown if he is still meta, only time will tell.
For Jg
WE ACTUALLY HAVE VARIETY NOW, ITS NOT LONGER PLAY THE SAME THREE CHAMPIONS OR UR A TROLL BS
Literally anyone can be played in the jg and I fucking love it the only thing you need to watch out for is 
xin cant be traded with in a one v one, he will always win it.
For mid
Honestly nothing has changed here and I feel the majority of my fellow players can agree
You play assassins when you wanna roam and carry the early game
You play mages when you wanna afk farm and carry the late game in team fights
For adc
THE RISE OF KALISTA
The best adc in the game by far once mastered,
Passive-every time u auto attack you are given the option to immediately click a area near you and hop into that direction, this gives her amazeing kite potential once you properly learn this mechanic
Q-Throws a spear that adds to rend stacks (we will explain those later)
W-Is a passive steroid while the active summons a liveing ward/ghost
E-Rend (rend stacks) for every auto you do you apply a spear stack, for every spear in your enemy when you press E you will do damage based off the ammount of spears that are in your enemy
R-Turn your support into a flying ghost thing that can not be target and the area your support choses to click on will do a aoe knock up to anyone hit by it
The current best adcs are
Kalista, Lucian, Kogmaw, Ezreal, corci (this always changes depending on the meta and what type of adc is needed, currently team fighting kite adcs are the best)
For Support
If you wanna play a tank
The rise of Tahm Kench
at one point was considered the best support in the game, he is currently nerfed and has fallen off but is still good
Q-A skill shot slow
W-allows you to either, A eat a ally and walk away to rescue them, B-Eat a minion and spit out out onto the enemy to do damage and apply stacks or C-Once the enemy has 3 stacks you can eat them to make them basicaly be stunned, as you walk away to controll where they will land and then do a burst of damage
E-A meter will show as you take damage, you are either allowed to do nothing , walk away and your health bar will recover by basically half of what you lost, or B press E and you will get a shield according to how much health you lost
R-Twisted fate ultimate, and allows you to scale your damage with Health items, you are basically the Health version of malphite (malphite scales with armor)
Meta supports
Tahm Kench
Alistar
Braum
Sorraka
Janna
For masteries
just look up a youtube video or have a friend teach you tbh, once you learn it you never forget it
Anyways good luck rejoining league :3 the reason top lane had the most detail for honestly top lane has received the biggest changes from season 5 to 6
is a thing, and will usually be in every game, your team needs one to counterpush the enemy rot or that lane will be permanently pushing to your side.
I may have missed some of the minor ones (help me out if I missed some guys!).
and he's glowing Do Not Go Near Him.