Why can't I attach document to email

Today i am unable to attach scanned documents to my emails. I click attach and attach files opens as soon as I try to click on the document required it says attach file (Not Responding). Then click on cross to close the program and is says Windows is checking for a solution and then Mozilla Thunderbird closes. I have tried switching computer off and on a couple of times. This has never happened before and i have several urgent documents to send. Thanks in advance.