The Employment Pass (EP) is cancelled immediately. You then have 30 days to find a new job and secure a new Employment Pass, failing which you will need to leave Singapore.
Suggested Readings
Can I become a director of other companies while I am on Employment Pass (EP) for one company?
Can I hold shares in other companies while I am on Employment Pass (EP)?
Can an existing Employment Pass holder be a director of another company in Singapore?
Do Singapore Employment Pass holders need to contribute to any social security or skill development fund etc?