Lord Ptolemy

Did you download a Minecraft jar then rename it to modpack.jar? That would be why your modpack doesn't work. The modpack.jar is suppose to be a mod loader like Forge.
 

Restart the launcher. An update was pushed to resolve the issue. You will have to reinstall the modpack.

The stable stream was rolled back to build #568 to temporarily resolve this issue. Any modpacks installed with this issue while using build #580 will have to reinstalled. You can do so by clicking on modpack options in the top right then clicking on the reinstall button. Then exiting the modpack settings and click the install button.
 
 
EDIT: Stable & beta streams were just updated to build #582. This resolves the issue. You will still have to reinstall the modpack.

Since this is still a common problem. All non-launcher bug reports posted here will be locked. I will no longer be moving them to the correct section. If it is blatant you will receive a warning, you may even have your posting privileges revoked for a period of time.
 
What counts as a launcher bug?
- The launcher not starting.
- Not being able to login on the launcher
- If ALL official modpacks fail to download.
- If ALL official modpacks fail to launch.
 
What is not a launcher bug?
- A community modpack not launching.
- A community modpack not downloading.
- Minecraft window looking weird or not responding.
- Minecraft Issues
- A mod not working in a modpack
 
I want to also make it clear that if you thread title is in reference to any non-launcher issues that I will lock the thread without bothering to read the body.

Moved to launcher bug reports. Update your launcher, this has been resolved.

Moved to launcher bug reports. Update your launcher, this has been resolved.

When signing into the Technic Launcher, you must have bought a copy of the Java edition of Minecraft as you must login with your Mojang account and NOT your Technic account.
1.) Log in with your Mojang/Minecraft EMAIL and password: https://help.mojang.com/customer/portal/articles/329527
2.) Try logging into your Mojang account: https://account.mojang.com/login
3.) Reset your Mojang password: https://help.mojang.com/customer/portal/articles/329524
4.) Contact mojang support: https://help.mojang.com/customer/portal/emails/new?ref=footer. We CANNOT help you any further. For those that want to make sure their information isn't being stolen. You can see the exact code used in the launcher here: https://github.com/TechnicPack/MinecraftCore/blob/master/src/main/java/net/technicpack/minecraftcore/mojang/auth/AuthenticationService.java#L97:L111

This is a bug with Java 8 on Windows 10 Anniversary that's apparently fixed in Java 9.
Not entirely sure if this will fix your issue as I haven't been able to get anyone to test it and give feedback.
 
1.) Download this version of the Technic launcher: http://launcher.technicpack.net/launcher4/349/TechnicLauncher.jar
2.) After downloading place that jar file on your desktop
3.) In the "Search Windows" bar type in "command prompt"
4.) Copy and paste this into the command prompt:
java -Dswing.defaultlaf=javax.swing.plaf.nimbus.NimbusLookAndFeel -jar %USERPROFILE%\Desktop\TechnicLauncher.jar 5.) Try navigating to the "Mod Options" tab in the Technic launcher again.
 
You can read up more on this issue along with what that command is doing by looking at these links:
https://bugs.openjdk.java.net/browse/JDK-8179014
https://bugs.openjdk.java.net/browse/JDK-8178873
http://bugs.java.com/bugdatabase/view_bug.do?bug_id=8178873
https://docs.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html#commandLine
 
EDIT:
I was able to get a confirmation that the above does not work. However the fix for this issue was back-ported to Java 8. Update your Java from this link:
https://www.java.com/en/download/manual.jsp
Get the one that says Windows Offline (64-bit)