How often does this happen? Is the service set to Automatic start after computer restart? When this happen go to Windows Event Viewer as shown below and you should find Netcam Studio there as an error. Read below why it stopped.
It can be something from the cams that make it freeze. Are the cams connected with rtsp och http?
Since you only have 3 cams I would just do a clean installation as from here How to make a clean installation of Netcam Studio
Before you install NCS again make sure that WIndows is completely updated.
-Henrik