Short answer. You risk arrest and contempt of court. The court may issue a warrant to have you arrested and brought before it, with the cost charged to you if your failure was willful and without just excuse, and disobeying a subpoena without adequate cause is itself deemed contempt of court.
What the law says
In case of failure of a witness to attend, the court or judge issuing the subpoena, upon proof of the service thereof and of the failure of the witness, may issue a warrant to the sheriff of the province, or his or her deputy, to arrest the witness and bring him or her before the court or officer where his or her attendance is required, and the cost of such warrant and seizure of such witness shall be paid by the witness if the court issuing it shall determine that his or her failure to answer the subpoena was willful and without just excuse.
Rule 21, Section 8 — Compelling attendance. Read the full provision →
What the law says
Failure by any person without adequate cause to obey a subpoena served upon him or her shall be deemed a contempt of the court from which the subpoena is issued. If the subpoena was not issued by a court, the disobedience thereto shall be punished in accordance with the applicable law or Rule.
Rule 21, Section 9 — Contempt. Read the full provision →
A warrant of arrest can follow a proven failure to appear
Once there is proof of the service of the subpoena and of the witness's failure to attend, the court may issue a warrant to the sheriff to arrest the witness and bring them before the court or officer where their attendance is required. This is a direct, coercive response to a witness who was properly served but simply did not show up as required.
A willful, unexcused failure means paying for your own arrest
The cost of the warrant and the witness's seizure falls on the witness personally, but only if the court issuing it shall determine that his or her failure to answer the subpoena was willful and without just excuse. A witness with a genuine, justifiable reason for not appearing is in a different position from one who simply ignored the subpoena outright.
Ignoring a subpoena is separately treated as contempt
Beyond the arrest warrant, the rule independently provides that failure by any person without adequate cause to obey a subpoena served upon him or her shall be deemed a contempt of the court from which the subpoena is issued. Contempt carries its own consequences apart from being physically brought to court, and applies broadly to disobedience of a subpoena, not only to a complete failure to appear, so a witness cannot avoid it merely by showing up eventually.
Non-court subpoenas are still enforceable, just under different rules
Where the subpoena was not issued by a court — for instance, one issued by an administrative body — the disobedience thereto shall be punished in accordance with the applicable law or Rule governing that particular body, rather than through ordinary court contempt. Ignoring a subpoena from a non-judicial source still carries consequences, just through a different legal mechanism specific to the body that issued it.