If you can't view attachments (and there's no security message about them being deleted because they contain malware) the most likely cause is either that
(a) your computer hasn't got a suitable program (e.g. someone has emailed you an Excel spreadsheet and you've not got Excel, or a compatible program, on your computer) ; or
(b) your computer has lost the 'file association' which tells it which program to use to open the files.
So what's the file extension (e.g. .doc, .docx, .xls, .ppt or .jpg) on the end of the file names of the troublesome attachments?
Further, what happens when you try to open the attachments?