What happened to Nausicaa at the end of the movie?
Strictly speaking, there is no way to know, as the creators have not said one way or another. However, we're led to believe the following (excerpt from Wikipedia):


   The herd, however, calms down, and the Ohmus use their golden tentacles to heal Nausicaä's wounded body.


Based on the use of the words "heal" and "wounded", it seems she never actually died.

Additionally, there is this bit from the Nausicaä Wiki:


   Nausicaä was badly injured trying to save her people from an attack by the Omhus. She stood before the rush of angry creatures, almost costing her her life. The Omhus, however, revived Nausicaä and she was brought back to conciousness.


Unfortunately, it's not cited to any specific source, but it seems to be the general concensus.