Java can become disabled in your broswer (maybe after a Windows update or other update), so here is how to check:
Checking the Java System Settings...
Now click the Advanced tab...
...open the entry marked Default Java for browsers,
...and make sure that the checkboxes in here are all ticked:
Java has been correctly installed and is enabled in your System Settings!
Try playing one of the Java browser games again, and if you still have problems then the issue may be with your Browser Settings instead.