Report the score as it stood when you stopped. If at least one set was completed, the team ahead counts as the winner and levels change by half the usual amount. If no set was completed, or the score was level, nobody's level changes.
How to report it
Tap the "Report the score" notification (or find it under the bell icon on the Profile tab), then tap Report score.
Tap Yes under Was this match played?.
Enter the games as they stood, including the unfinished set.
Submit the score.
Don't make up a final score. Report what was actually played.
How it affects levels
At least one set completed: the team with more sets (or, if equal, more games) counts as the winner. Level changes are half the usual size.
No set completed, or exactly level: no level change.
Not played at all: tap No under Was this match played?. No level change.
Good to know
An unfinished match doesn't count as a win or a loss in your match stats.