If you are away on Polling Day, you will not be able to cast your vote. Your name will be removed from the Registers of Electors after the election. 

You may apply to restore your name to the registers at Voter Services using your Singpass. Being overseas is regarded as a valid reason for not voting and there will be no administrative fee levied when you apply to restore your name to the register.

The source of information is from the ELD website.