Forgotten belote: you lose it
The belote must be called out loud as you play it. Forget it and it's lost: no 20 points.
Reminder: what the belote is
The belote is holding the King and Queen of trump in the same hand. It scores 20 points, provided you announce it. You say "belote" as you play the first of the two cards and "rebelote" as you play the second.
Forgetting = lost points
If you play your King and Queen of trump without saying "belote" and "rebelote" at the right moment, you lose the 20 points. The belote isn't counted automatically: the announcement is what validates it. Claiming it afterwards is generally not accepted.
The right habit and timing
Say "belote" exactly as you lay down the first of the two cards (King or Queen, in whatever order you play them), then "rebelote" on the second. You needn't play both in the same trick, but both calls must be made to validate the 20 points. It's a common slip that can cost a tight deal.
See also
FAQ
What happens if you forget to announce the belote?
The 20 belote points are lost. The belote is only valid if announced out loud as you play the King then the Queen of trump.
When do you say belote and rebelote?
You say belote as you play the first of the two cards (King or Queen of trump) and rebelote as you play the second. Both calls are needed to score the 20 points.