Quartus II error, cann't open *.bdf file

I use Quartus II 9.0 as the development tool and choose the BDF as the design input. At the begin, everything is fine and I have complteted the design and perfromed the simulation by simulator of Quartus II. However, the quartus output one warning message (Error:Cann't open file E:\hardware\CPLD\design.bdf and Cann't find requested location) after one time of synthesis. Then, I closed the project and open the project again. The BDF file can not be opened. The error message is shown as above. Is there any solution?

Thanks!