What version of FileMaker?
What storage options were selected in Manage | Database for this field?
What exact method and options were used to insert the PDF?
From what you are reporting, I would guess that the file was inserted with the "store a reference" option selected. In such cases, the file is not actually inserted into the field. Instead, the file path to that file is inserted. And then, anything that moves, renames the file or any volume/directory in the file path will produce the result you see.
I am using filemaker pro 12 advanced.The method i used was via insert pdf...It was there few days ago..But its vanished another day...
What do u means by storage options?
Open Manage | Database | Fields
Double click the field to open field options.
Click the storage tab
What options are specified there?
When you inserted the file, did you select the "store a reference" check box?
After inserting the file, what did you do with that file? Did you move it? Rename it? Did you make any changes to the folder(s) it was inside of? Was it inserted from a shared directory?