ical error and crash

Everytime I open iCal, I automatically get the following error message:

"The server responded with an error.

The server did not recognize your user name or password for account “XXXXX@gmail.com.” Make sure you enter them correctly."

When I press OK, the error message pops up againg. And then when I press OK for the second time, iCal crashes and I receive the following message:

"iCal quite unexpectedely."

I have reopened iCal 10 times now with no luck. Same thing happens each time.