It depends on why the witness is not there, what kind of witness it is, what kind of case it is, and what sort of evidence the witness is expected to give.
In a criminal trial, the case may be dismissed if there is no witness. Or the trial may be delayed while law enforcement finds and brings the witness to court (if they are ignoring a subpoena). If the witness is ill or travelling, the case may be continued to wait for the witness. If the witness is not key, the parties may proceed without him/her, or they may shift around the order or witnesses to allow that witness to appear later in the trial.
sworn
has sworn
No, a trial begins when the bailiff has declared the trial to be in session, and the judge taps his gavel.
The bailiff swears in the witness during a trial. The witness holds their right hand on the Bible and their left hand is in the air.
Jury
the final round of questioning of a witness in a trial
no
In most jurisdictions, the person making the arrest merely completes an affidavit (sworn statement) for the court, and appears in person only if the arest is contested at trial and they are called as a witness.
Perjury
the notery and witness by
Are you asking about a witness?
On Trial - 1956 Strange Witness - 2.5 was released on: USA: 3 August 1959