Mixer design issues in ADS

I am designing a single FET mixer using ADS and I had designed the matching circuit at the required frequency by placing the smith chart matching smart component and designing the circuit using sith chart utility. the mixer was giving a decent gain for a single ended mixer during simulation. the next time I opened the software,,,the smart elements were missing and the strange part is i can still see an empty box in place of the smart component and its name in the schematic window....and i get an error that says:

Can't get item definition for D:\Dissertation_mixer_prj\networks\DA_SmithChartMa tch_2conv_gainandIF.
Connectivity checker reported a connectivity error.
(..\..\..\..\projects\physsim\ael\mom\momServer.ae l) D:\Dissertation_mixer_prj\networks\2conv_gainandIF
Could not netlist design

and when i looked through the list in the project file i could not find the file associated with the smat component....is there anyway I can retrieve it? and I did not delete it so I really dnt knw waht happnd to it....moreover when I am designing using the same parameters as before I am not getting the same reuslts...any help would be appreciated as I have submit the project real soon..