In short you need to attend.
At the Crown Court the trial will be re-run but this time before a bench consisting of a Crown Court judge and two magistrates (but obviously neither of them will have any knowledge of the earlier proceedings).
If you do not attend the prosecution may ask the court to issue a witness summons against you. If you do not attend in response to that you may be arrested and brought to court in custody. Crown Court judges take a very dim view of people refusing to respond to orders from the court and you may find the consequences somewhat serious. Any action taken against you will almost certainly appear on Disclosure and Barring check (this is the new name for a CRB check).