Parastoo is right: Both are correct, but they are correct in different contexts. Here, I'll give a few examples to differentiate their use:
(1) Maybe ("Perhaps," used as an adverb)
I wonder why it's taking John so long to get here? - Maybe he got lost.
Maybe Sara will regret her decision and take you back as her boyfriend.
(2) May be (this is basically the modal verb "may" plus the infinitive "be," so this would be the verbal phrase of a sentence)
I may be going to London on business next week!
I may be overly cautious, but I think you should go to see the doctor.
Basically, if you can replace "maybe" with "perhaps" in your sentence, then spell it as one word.
X I perhaps going to London on business next week --> wrong, so two words
O Perhaps he got lost --> right, so use one word!