Why did this script break suddenly?

First: tab your code correctly. There are a few lines it's off. You should also keep space around your operators; a == b is a lot easier to read than a==b, the same goes for a = b. There's also no reason to put parenthesis around your strings..

Now look at how many times Loading.Visible = false happens. It is written 15 times, even though it can just go once at the end.

Now notice that you continually say x.Visible = true for different x. The x only depends on what num is, which is 1, 2, 3, 4, .... This suggests we should probably be getting these xs from lists.

results.Changed:connect(function()
    local value = results.Visible
    wait(1)
    Loading.Visible = true
    wait(3)
    if stepone.Text == "Action & Adventure" then
        if steptwo.Text == "Easy" then
            local options = {TMM, LD}
            local num = math.random(#options)
            options[num].Visible = true
        elseif steptwo.Text == "Medium" then
            local options = {AR, AFSS, NDS, TQ}
            local num = math.random(#options)
            options[num].Visible = true
        end
    elseif stepone.Text == "Building & Comedy" then
        if steptwo.Text == "Easy" then
            local options = {SBS, S, WTRB, PIR, HITW, DI, BB2}
            local num = math.random(1, #options)
            options[num].Visible = true
        elseif steptwo.Text == "Medium" then
            local options = {PTP, D2}
            options[num].Visible = true
        end
    end
    Loading.Visible = false
end)

Now we are repeating ourselves again:

local num = math.random(#options)
options[num].Visible = true

The thing that is different here is options, so we just need to select that.

results.Changed:connect(function()
    local value = results.Visible
    wait(1)
    Loading.Visible = true
    wait(3)
    local options -- Declare options
    if stepone.Text == "Action & Adventure" then
        if steptwo.Text == "Easy" then
            options = {TMM, LD}
        elseif steptwo.Text == "Medium" then
            options = {AR, AFSS, NDS, TQ}
        end
    elseif stepone.Text == "Building & Comedy" then
        if steptwo.Text == "Easy") then
            options = {SBS, S, WTRB, PIR, HITW, DI, BB2}
        elseif steptwo.Text == "Medium" then
            options = {PTP, D2}
        end
    end
    local num = math.random(#options)
    options[num].Visible = true
    Loading.Visible = false
end)

Actually, we can eliminate the ifs and elseifs altogether. We can just make a dictionary with two levels:

local data = {}

local action = {}
action.Easy = {TMM, LD}
action.Medium = {AR, AFSS, NDS, TQ}
data["Action & Adventure"] = action

local building = {}
building.Easy = {SBS, S, WTRB, PIR, HITW, DI, BB2}
building.Medium = {PTP, D2}
data["Building & Comedy"] = building

results.Changed:connect(function()
    local value = results.Visible
    wait(1)
    Loading.Visible = true
    wait(3)
    local options = data[stepone.Text][steptwo.Text]
    local num = math.random(#options)
    options[num].Visible = true
    Loading.Visible = false
end)

-- 79 lines have become 22: cut out more than 2/3 of it
-- leaving much less that you have to be able to understand

Though I am now assuming that stepone.Text actually is a valid option. If it's a TextBox, that might not be true (they could mistype it). Though, expecting the player to accurately type "Building & Comedy" probably isn't a good choice.

What does the output say?