Help Excel Has stoped working!!!

Hi Everyone,

I have a big problem,
My excel spreedsheet has stoped working.
It has several formulars in it and has worked fine up until yesterday, then it just stoped.
None of the formulars now calculate.
they will start working again if i click n the cell with the formular in it then click the tick box but i cant do this for every formular on the page.

anyone know whats causing this?

thanks

Tony