Stocks are delisted for multiple reasons. You may monitor the ASX announcements from the companies you own stock in. 

Most often the company will make an announcement regarding the delisting, however once a company has already been delisted you will no longer be able to search for the stock on Stake.

If the company has been delisted as a result of a Corporate Action, you may get cash, shares or warrants as per the terms of the agreement. 

If a company goes into liquidation, bankruptcy or is suspended from the ASX, you still have full legal and beneficial ownership of the shares that you hold in the listed company. 

You can search for delisted companies via http://www.delisted.com.au

Once delisted, it is best to reach out to the share registry directly for advice as to how to proceed.