Excel Web Access Error message

I use SharePoint 2007 and have used Excel Web Access to link to a spreadsheet for a few months. It's a rota so is updates every few months and all I do is change the file path to match the new file being used. I did this on Friday and it worked fine, but this morning (Monday) I am getting the error message "An erro as occurred". I have tried a new webpart but get the same message, I have tried reverting back to the old rota which worked once and then when I tried it again I got the same error message, now whatever I do I get the error message. I'm not sure why it's not working as nothing has changed over the weekend. What do I need to do to get it to work again? Thanks