Why are my datastores not working? They worked yesterday.

Start by copy and pasting this into your script, I will explain once you're done:

local StoreService = game:GetService("DataStoreService")
local DataStore =  StoreService:GetDataStore("your_data_store_here")

local function PlayerJoined(User)
    local leaderstats = Instance.new("Folder") -- it's bad practise to set parents in the brackets.
    leaderstats.Name = "leaderstats"
    leaderstats.Parent = User

    local Value1 = Instance.new("IntValue") -- don't call it value1, it's just an example.
    Value1.Name = "Value1"
    Value1.Value = 0
    Value1.Parent = leaderstats

    local Value2 = Instance.new("IntValue") 
    Value2.Name = "Value2"
    Value2.Value = 0
    Value2.Parent = leaderstats


    local UserId = "User-"..User.UserId
    local data = DataStore:GetAsync(UserId)

    if data then
        Value1.Value = data["Value1"]
        Value2.Value = data["Value2"]
    else
        Value1.Value, Value2.Value = 0, 0
    end
end

Essentially what's going on here is I make a function which holds values just like this. You SHOULD know how userids and getting data works..GetAsync..

Now, look below and copy it (make a new 2-3 lines before doing so):

local function create_stat_table(User) -- We want to get the children of leaderstats.
    local user_stats = {}

    for _, stat in pairs(User.leaderstats:GetChildren()) do -- for everything in leaderstats, do:
        user_stats(stat.Name) = stat.Value 
    end
    return user_stats -- returning the values will help when we call the function.
end

The PAIRS statement just finds any children of "leaderstats", then if it does it will add THAT child into the user_stats_ table:

--[[
when you do "user_stats(stat.Name) = stat.Value, it would look like this:

Computer: "....Oh, so I add a new variable with a value!:"

user_stats(stat = value) 

!!STAT AND VALUES ARE JUST EXAMPLES, stat would be the name of the stat, and the value would be the value of the stat!!



]]--

Now, let's do the player removing function (same as last time, 2-3 new lines):

local function PlayerLeft(User)
    local user_stats = create_stat_table(User) -- the returned values will replace the function.

    local success, err = pcall(function()
        local UserId = "User-"..User.UserId
        local data = DataStore:SetAsync(User.UserId, user_stats)
         -- you set the values with the table.
    end

    if not success then
        print(err) -- print what error has happened if it fails
end

The pcall functon is just for error handling; you can check if the code within this code block errors or not.

GetAsync: Fetching values, good for checking if they have existing data or not. SetAsync: Changing values to whatever we want, in this case we want it to change is maybe Coins, etc.

FINALLY, make 2-3 more lines again and do:

game.Players.PlayerAdded:Connect(PlayerJoined) -- connecting function to event
game.Player.PlayerRemoving:Connect(PlayerLeft) -- no need to call table function here, we've already called it.

I hope this clears up what was wrong, if not please reply and I'll try my best to explain! :3

P.S don't always use functions for one-time-use events, such as a player joined event. it's not very good practise..