Rails - May 2006 - inexplicable undefined method errors

Hi All,

I had a function from the pragprog book in my application_helper.rb  
file. Here it is:

   def format_price(amount)
     dollars, cents = amount.divmod(100)
     sprintf("$%d.%02d", dollars, cents)
   end

I call this function in my view quite simply like this:
<td><%= format_price(hard_product.price_ca) %></td>

This was working fine until today, suddenly for now apparent reason,  
it gives an error:
undefined method `format_price'' for
#<#<Class:0x267de2c>:0x267ddc8>

Even when I move this function to the controller for my view I still  
get the error. But I can''t imagine why. Any ideas? I''ve tried
reloading the server, etc. but something else must be going on here.

Thanks,
Sean