Re: Going back to training
Unless your reason for leaving involved some heated argument with your sensei, you are probably being overly worried about it. Just some offhand remarks like "it has been way too long..." and "I forgot how much I enjoyed this!" ought to be enough.
If you did leave after an argument, be genuinely humble and repentant. Keep an apology short and simple; being long-winded or making a spectacle of yourself is false humility.