Yup, very first port of call (after a wee bit of penetrating oil) is to try and “unscrew” the rotor. That’ll either turn the bolt, or if it turns without the bolt, at least you know the bolt’s not seized onto the disc (and if it doesn’t turn at all you lose nowt)
It’s threadlocked in generally so a bit of heat is very helpful. Cutting a slot into the bolt with a dremel will add heat if you’re not happy about blowtorching it. But if you’re not comfortable with this sort of destructive stuff, it’s not a bad idea to give it to a (good) LBS as some of the tricks are one-shot and if you screw it up they can’t do it at all.
Oh and as a general rule, make sure there’s no dirt in the bottom of the bolt hole- not to say there was this time but it’s a big cause of rounded torqs.