What will happen, if default init level set to 6 in linux?

if  init level set to 6 i.e default run level is 6 . your System keeps rebooting. To recover from this you need to switch to emergency mode while machine boots. and change the default boot level to 3 or 5 whichever applicable.

N.B. if the default run level is set to 6 , your system will go upto init.rd and then will jump to reboot without loading kernel.