Yes, you can still divorce.  The courts have created a method of dealing with this problem.  This is called a ‘Publication Divorce’.  The divorce is filed in the courts in the usual way.  In order to serve the spouse, a search needs to be performed in an attempt to locate the missing spouse.  Once the search is complete, a publication in a certain newspaper takes place.  When this is complete, one either finds the missing spouse or does not.  If the spouse is found, then he/she can be served in the usual way.  If the spouse cannot be found, then the paperwork is prepared and divorce is completed through the court system.