Ok, here's what happened:
I was playing on my creative map, when suddenly, the game randomly crashed after some chunks didn't load and I placed blocks in them to update them. I had no idea what happened. I boot up Minecraft again, and the world is gone on the list of available worlds. I check the world folder. No level.dat is present. The report of the crash can be found below. Anyway, I hope someone can save the world I made, because I worked REALLY hard on it. If you do happen to be able to help me, I may release it as a map. Alright, now for that crash report:

---- Minecraft Crash Report ----
// Don't do that.

Time: 4/18/18 7:34 PM
Description: Exception generating new chunk

java.util.concurrent.ExecutionException: java.lang.RuntimeException: We are asking a region for a chunk out of bound | -20 -29
at java.util.concurrent.CompletableFuture.get(CompletableFuture.java:2237)
at sg.c(SourceFile:129)
at sn.a(SourceFile:84)
at so.c(SourceFile:143)
at sh.k_(SourceFile:219)
at net.minecraft.server.MinecraftServer.w(SourceFile:694)
at net.minecraft.server.MinecraftServer.v(SourceFile:627)
at dal.v(SourceFile:155)
at net.minecraft.server.MinecraftServer.run(SourceFile:532)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.RuntimeException: We are asking a region for a chunk out of bound | -20 -29
at sm.c(SourceFile:114)
at sm.a(SourceFile:100)
at sm.b(SourceFile:120)
at sm.c(SourceFile:125)
at brj.a(SourceFile:87)
at brn.a(SourceFile:138)
at brn.a(SourceFile:116)
at brn.a(SourceFile:104)
at brn.a(SourceFile:61)
at brn.a(SourceFile:18)
at bro.a(SourceFile:26)
at bio.a(SourceFile:60)
at ss.a(SourceFile:13)
at st.a(SourceFile:33)
at biq.a(SourceFile:85)
at sz.a(SourceFile:54)
at sz.a(SourceFile:23)
at zx$a.a(SourceFile:143)
at zx$a$$Lambda$1016/1643506556.apply(Unknown Source)
at java.util.concurrent.CompletableFuture$AsyncApply.exec(CompletableFuture.java:501)
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:902)
at java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1689)
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1644)
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

– Head –
Thread: Server thread
Stacktrace:
at sg.a(SourceFile:178)

– Chunk to be generated –
Details:
Location: -16,-33
Position hash: -137438953488
Generator: bke@518d2001
Stacktrace:
at sg.a(SourceFile:178)
at sg.c(SourceFile:132)
at sn.a(SourceFile:84)
at so.c(SourceFile:143)
at sh.k_(SourceFile:219)
at net.minecraft.server.MinecraftServer.w(SourceFile:694)
at net.minecraft.server.MinecraftServer.v(SourceFile:627)
at dal.v(SourceFile:155)
at net.minecraft.server.MinecraftServer.run(SourceFile:532)
at java.lang.Thread.run(Thread.java:745)

– Affected level –
Details:
Level name: Slomo Island
All players: 1 total; [sj['TLORD765'/222446, l='Slomo Island', x=-50.86, y=68.37, z=-717.42]]
Chunk stats: ServerChunkCache: 999 Drop: 0
Level seed: 27594263
Level generator: ID 02 - largeBiomes, ver 0. Features enabled: true
Level generator options:
Level spawn location: World: (0,62,0), Chunk: (at 0,3,0 in 0,0; contains blocks 0,0,0 to 15,255,15), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Level time: 943294 game time, 29875 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 13495 (now: true), thunder time: 81338 (now: false)
Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: true

– System Details –
Details:
Minecraft Version: 18w15a
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 352458048 bytes (336 MB) / 1006632960 bytes (960 MB) up to 1073741824 bytes (1024 MB)
JVM Flags: 8 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=16M
Profiler Position: N/A (disabled)
Player Count: 1 / 8; [sj['TLORD765'/222446, l='Slomo Island', x=-50.86, y=68.37, z=-717.42]]
Data Packs: vanilla
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.

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