Error Message

I've been working on a 21 board portfolio for the last few days and the file itself isn't even that big to be honest but last night when trying to save it, the error message 'Acrobat PDF file format is having difficulties. A number is out of range'' kept appearing and stopping me from saving the file, but I've not once tried to save it as a pdf. only ai. file. Since then the entire Illustrator programme has crashed and I've lost the majority of the work but when I set up a new file to start again and I get to around board 9/21 and try to save the file (again just as a plain ai file) the same message keep popping back up and I can't work out why.
Any help or suggestion as to why/how to stop it would be appreciated.