Trouble shooting
If you were getting 20+ channels and now you are only getting 1 or 2, you either have a bad converter box or the box is not getting a signal. To me it sounds like the antenna is no longer connected and the box is only using the cable to as an antenna. The pre-amp can only work with the signal it gets, and the more distance between it and the antenna. A antenna that has been up for years may very well have lost a connection between it and cable. If you try a new converter box and that does not change things, then you will have to verify that the connection between the antenna and the converter box is solid. The pre-amp should be at the antenna to get its maximum value.
Another thing to consider is when did you lose the other stations. Something happened when the signal was lost, be it a connection or a converter box failing.