If you've been requested to attend court as a defendant and you don't, it's very likely a warrant will be issued for your arrest.
If you've been bailed to attend court (by the police or the court itself), failing to attend your hearing without a valid reason is a criminal offence.
It's likely you'll be charged for this offence as well as being charged for the original reason you were required to attend for. The court will issue a warrant for your arrest.
When arrested, you'll be kept in police custody until you can be put before the next available court who can carry out both your original hearing and the matter of your 'failure to appear'.
The court will consider a separate sentence for your 'failure to appear' irrespective of the outcome of the original offence you were charged with.
If you've been issued with a requisition or summons to attend court and you do not attend, although no offence will be committed, the court may still issue a warrant for your arrest.
In certain circumstances the court may go on to prove the case against you in your absence resulting in a conviction. When this happens they may issue a warrant for your arrest prior to a sentence being given.