Nothing happens normally and the plan just continues on. I had a look at the Critical Information Summary and nothing stands out. Were you wanting to change plans?
Just to make sure. You get and pay the same as before the plan ran out. You are now able to sign up to any new plan if you see one you like without penalty (or leave Optus if you want).
Note more recently Optus has started offering 'sweetners' to new plans which expire after 12 months. These plans would then change after that period (for example double data might diasappear). They're not a bad deal per say, just keep an eye out for them.