Mojang can't help if it has to do with mods, you probably installed the mods incorrectly, look up a tutorial on Youtube, if you can't seem to get it working after researching a bit and trying stuff, contact the mod creators, or the modpack creator.
This has happened on my Mac before, I just restarted my computer and it was fixed, try that. If not, reinstall Java unless you have the new launcher.
Mojang can't help if it has to do with mods, you probably installed the mods incorrectly, look up a tutorial on Youtube, if you can't seem to get it working after researching a bit and trying stuff, contact the mod creators, or the modpack creator.