Probably u were attacked. You have to turn off the warning message that comes after you were attacked. Depending on ur browser settings or sometimes if u r using anfits mod options that warning may not pop up.
Go to `Build Ships`. Then select one ship type to go to the page with single ship information.. (With the description, Hull, PR costs and all that) Then type the ship number and click build. Now the error message should be shown on screen... Click it to open a pop up. after it loads press close...
Then u can build....
This usually solves the problem, when the same thing happens to me...