You are simply marked absent — there is no penalty and no action against your candidature.
The exam body does not conduct a re-exam for candidates who miss the scheduled date.
The application fee is not refunded when you remain absent.
In exams with attempt limits, like UPSC, an attempt is counted only if you appear in at least one paper.
You can apply again in the next cycle without any disadvantage.
Reach the centre well in advance on exam day — late entry is not allowed in most exams.