In episode 1.12, she was killed by the Master and revived by Xander, and that was long enough for Kendra to be called, so Buffy was dead.
In 5.22 She died and was ressurected (three months later) by Willow.
She may also have died from the gunshot would in Season Six, wasn't she flatlining when Willow took the bullet out and resussitated (sp?) her.