Helep with "ActionController::InvalidAuthenticityToken"

hi all !
i have a rails app and it worked very well one day before.when i turn on
my computer next time i have got this error

ActionController::InvalidAuthenticityToken

every time i try to process the form .also i have tried to change the
form but the changes not available to see .even i start the server again
or restart my computer .have any idea .
thank you !
my rails verstion 2.1
gem version 1.1.1
ruby 1.8.6