well if you have a T-bred 2400+ XP default frequency should be 2GHz, it it's a mobile or barton core ( which I think the barton for 2400+ is somewhat more rare than a barton 2500+ 2600+) then I suppose it was 1.8GHz...
Anywho man...think about it.. a 1.2GHz OC?? That's hefty and if you don't have a minimum of watercooling then you shouldn't be goin for those speeds.
I ran my XP-M at 2.8GHz with 2.225 Vcore and it somehow survived that
If you've already cleared your CMOS and it still doesn't want to boot up then yeah I'm afraid you've probably fried it.