What happens or is done after cardiac arrest, at the hospital?

ACLS.... Cardiac arrest is when the heart stops beating. When this occurs, advanced cardiac life support protocols are followed and include cpr, administration of medication and converting the heart rhythmn by "shocking" the heart. This is initiated (hopefully) wherever the cardiac arrest occurs and the effort is continued in the hospital once the patient is transported there.
Depends. If the patient wants everything, the acls protocol is initiated, and (depending on the cause of the arrest and type of arrest) chest compressions, drugs, shocks, are given as appropriate. The patient is usually intubated or bag masked to get oxygen in. The pH of the blood or electrolytes may need to be corrected....
Hypothermia. It depends on several factors including witnessed/unwitnessed, time to presentation, observed initial rhythm, neurologic status (coma or no, or bleed), and of course cause of arrest (e.g., cardiac, respiratory). That being said...Hypothermia protocols are a very important part of treating cardiac arrest patients and have been shown to improve survival and neurologic outcome.