Are you trying to ‘Open from URL’ or from a local file? If from URL (or if this is an imported ontology), than the URL that appears in the dialog in the screenshot cannot be resolved. If you’re trying to open from file, then please supply the ontology so that we can investigate.
Many thanks for your reply. I am opening it from a local file I already modeled. Actually, I've imported some other existing ontology standards in my file as well like SSN Ontology, Geo Ontology, Geosparql ontology, MUO/UCUM ontology. Even if you simply import SSN ontology in your file from the URL given below
it still gives the same error. But I'm stunned that it never gave this error before. Now I don't understand what actually caused this loading problem.
Well, if you are using Protege 4.1/4.2/4.3 versions you should be able to import the SSN ontology in your file but with the said loading error. I guess the ontology still exists but some of the entities can't be seen now which used to be there before, it's very strange. Anyhow, many thanks for your interest and replies.If you get any clue in future please let me know.