The verb "mögen" can cover multiple meanings, in addition to the common case of expressing desire ("to like something"). The meaning used here is actually the first one describe in the Duden entry for "mögen":
Zum Ausdruck der Vermutung; vielleicht, möglicherweise sein, geschehen, tun, denken
Was mag er wohl denken?
Es mochten dreißig Leute sein (es waren schätzungsweise dreißig Leute)
An English equivalent might be (pun intended) something like "may" or "might", as in "What might he think?" or "There may be thirty people."
Was mag dies für Folgen haben?
could be translated as
What may be the consequences of this?
In the example sentence, the predicate is in past tense, so literally
What might be the consequences of this?
(I had to look up myself that "might" is past tense of "may".)
A more common way of saying it in English would be
What may have been the consequences of this?
Now for the "nun" (sorry, I'm a bit punny right now ;) ). "Nun" can, in addition to denoting a point in time, be used as a particle. It's not really unequivocal which of the meanings the Duden mentions is used here, but I'd go with number 6:
Situationsbedingt emotional verstärkend als Ausdruck der Ungeduld, Befürchtung, Enttäuschung o. Ä.
Kommst du nun mit oder nicht?
To get a grip of the meaning in the example sentence, imagine a task you have to execute. But while doing the task, you run into obstacle after obstacle. At the umpteenth obstacle, you might groan something like "Okay, what now?" This would be equivalent to German "Okay, was (ist) jetzt?" or "Okay, was (ist) nun?". To emphasize that this is the latest obstacle in a (long) row, you might add "wieder", "again": "Okay, was (ist) jetzt wieder?" or "Okay, was (ist) nun wieder?".
So, a complete translation of the example sentence could be
What might have been the consequences of this again?