I haven't heard of this problem before. It doesn't mean it hasn't happened but I don't think it is a common problem. Do you have the manual? And have you checked the voltages around U3? I'm assuming you can hear some signals (although faint) and that you have checked those obvious things (that we all forget) like headphone volume controls (if applicable), etc.. I don't know how long you had it stored away and there's always the possibility that something component failed when you turned it back on. It would be odd though. Do you have anything to trace the signal through the receive section of the rig? These radios aren't real complicated so it should be fairly easy to find where you are losing the signal (or not amplifying it).
Also there is a Yahoo Sierra user group at http://groups.yahoo.com/group/sierra_wilderness_radio/
If your not a member, you should join.