All the examples we've seen up to now have been of
conditional sentences that were not negative, but negative
conditionals are certainly possible. It's possible to have
conditional sentences with negative conditions, with
negative results, and with both negative conditions
and results:

If he isn't here, he's sick.
If he isn't here, he isn't feeling well.

If he doesn't know the answer,
he admits it.
If he doesn't know the answer,
he doesn't admit it.

If they aren't here, we'll miss them.
If they aren't here, we won't miss them.

If they don't attend the meeting,
I'll be surprised.
If they don't attend the meeting,
I won't be surprised.

If he weren't here, we'd call him.
If he weren't here, we wouldn't call him.

If he didn't know the answer,
he'd admit it.
If he didn't know the answer,
he wouldn't admit it.

If he hadn't been here, we would've
called him.
If he hadn't been here, we wouldn't have
called him.