Happened to me once or twice. If you really-really want it and you're well versed on xml and understanding of how boinc and web in general works, you can manually recreate it and have it ran. I did that successfully in my case. However, this is tech-savvy twiddle-fu and it's easy to mess it up, so I'm not going to post any instructions. I mean, a person that can do it without messing up is also a person that can figure it out on their own.
Also, there's no real need to do it, the unit will just time out and get recycled on its own.