If a class is abstract that implies the class is not fully implemented. Typically at least one method in the class is abstract too.
You declare it abstract to enforce the implementation of the method before the class ca be used. So you cannot instantiate an abstract class, and logically you cannot invoke an instance method in an abstract class, irrespective of the method be abstract or not.
Of course, if you declare the method as static you can invoke without any problem. As long as you are not suggesting an abstract static method, which by chance is not possible either, since there is no way to inherit an static method, and therefore there is no point in declaring it abstract.
Now, you could create an anonymous class that implements the method in abstract class and this way use it.
You coud even do it this way:
They weren't very bright, but they were very, very big. Ad contrast: