A CRJ-200 First Officer described being alerted to a track deviation by ATC; then realizing the FMS had lost its position information. A successful reprogramming followed; and they resumed on course.

Narrative

While at cruise [we] received a call from ATC asking if we had been assigned a hold or something by an earlier controller. We responded no and realized that the aircraft was in a slow left turn off course. We told the Controller that we would go direct [to an on-course] intersection as it was the first fix on our FMS. The Controller then told us to go to a fix further down our course. We then turned for this new fix. We then worked on the FMS trying to figure out what had happened. We looked at the position page to see where the aircraft thought it was and its location was all dashes. The plane seemingly had no idea where it was; like a GPS glitch or the FMS had a glitch. No error message was ever seen by me. ATC later asked what had happened and I informed them that the plane had lost track of where it was - we reprogrammed its position and everything seemed to be working. The flight continued on with no more difficulties. At the time of the event I was doing revisions to my flight manuals. I should have looked up more often to track where the flight was. I knew there was a slight turn coming up so I thought that was what the plane was doing. This was my first time having a correctly programmed FMS lose track of where it is. The best thing I can do is not trust that the FMS will always do what it is programmed to do. I need to pay closer attention to all turns and tracks of the airplane.