Choi, I have far too often had EXACTLY this same problem, with exactly the same result. Star Lost and then of course the guiding stops. In addition I have had other other problems, that also seem to revolve around some software issues with the ASI120mm mini.
As naakada points out there is a FW update on the ZWO software site for the ASI120mm mini. I am unsure from the description if the FW update specifically addresses this problem as well as the other issues I have been having, but it at least sounds like that is what it's intended to do.
I installed that update a few days ago on my ASI120MM but unfortunately mostly cloudy skies since then have prevented me from verifying if it has resolved this very annoying problem.
In my case the camera problems seems intermittent. I can go decent periods with very good guiding, but once the problems start to show up the guiding becomes impossible from that point on.
If you do update your camera would you please post your results here? Based on my local weather forecast it maybe quite some time before I can test my camera update and see if my issues are resolved. Thanks Al