I had a MC account known as "Megapile" and whenever I tried to log in it told me that my user did not exist, but only on Multiplayer. So I registered and it worked. I was still logged into my old MC account in the start-up menu and I tried logging in with my new credentials and they didn't work either. Please help with this issue as I have no backups of server IPs. Thanks!
Crash Log: ---- Minecraft Crash Report ----
// Quite honestly, I wouldn't worry myself about that.

Time: 1/3/16 4:01 PM
Description: Rendering screen

java.lang.IllegalArgumentException: bad Base64 input character at 1620: 0 (decimal)
at io.netty.handler.codec.base64.Base64.decode(Base64.java:247)
at io.netty.handler.codec.base64.Base64.decode(Base64.java:197)
at io.netty.handler.codec.base64.Base64.decode(Base64.java:188)
at azk.c(SourceFile:212)
at azk.a(SourceFile:145)
at awd.a(SourceFile:25)
at awi.b(SourceFile:412)
at awi.a(SourceFile:211)
at azh.a(SourceFile:291)
at bfk.a(SourceFile:1039)
at ave.av(SourceFile:915)
at ave.a(SourceFile:325)
at net.minecraft.client.main.Main.main(SourceFile:124)

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

– Head –
Stacktrace:
at io.netty.handler.codec.base64.Base64.decode(Base64.java:247)
at io.netty.handler.codec.base64.Base64.decode(Base64.java:197)
at io.netty.handler.codec.base64.Base64.decode(Base64.java:188)
at azk.c(SourceFile:212)
at azk.a(SourceFile:145)
at awd.a(SourceFile:25)
at awi.b(SourceFile:412)
at awi.a(SourceFile:211)
at azh.a(SourceFile:291)

– Screen render details –
Details:
Screen name: azh
Mouse location: Scaled: (293, 141). Absolute: (586, 196)
Screen size: Scaled: (427, 240). Absolute: (854, 480). Scale factor of 2
Stacktrace:
at bfk.a(SourceFile:1039)
at ave.av(SourceFile:915)
at ave.a(SourceFile:325)
at net.minecraft.client.main.Main.main(SourceFile:124)

– System Details –
Details:
Minecraft Version: 1.8.9
Operating System: Windows XP (x86) version 5.1
Java Version: 1.7.0_15, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
Memory: 60552752 bytes (57 MB) / 233107456 bytes (222 MB) up to 523501568 bytes (499 MB)
JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx512M -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
Launched Version: 1.8.9
LWJGL: 2.9.4
OpenGL: GeForce 8800 GT/PCIe/SSE2 GL version 3.3.0, NVIDIA Corporation
GL Caps: Using GL 1.3 multitexturing.
Using GL 1.3 texture combiners.
Using framebuffer objects because OpenGL 3.0 is supported and separate blending is supported.
Shaders are available because OpenGL 2.1 is supported.
VBOs are available because OpenGL 1.5 is supported.

Using VBOs: No
Is Modded: Probably not. Jar signature remains and client brand is untouched.
Type: Client (map_client.txt)
Resource Packs:
Current Language: English (US)
Profiler Position: N/A (disabled)
CPU: 1x Intel(R) Pentium(R) 4 CPU 3.40GHz