The clock/weather widget is part of the HTC ROM so it can't have been deleted. It should still be there, somewhere...
Another place to look is Menu > Personalize > Widget > Clock > and look through the different clocks widgets there.
You've probably already tried turning the phone on and off, but you can also try taking the battery out for 30 seconds. This is a 'soft reset' and does more than switching the phone off does. Things often re-appear or reset themselves after doing this.