you can't 'erase' any synth as they are in ROM where you have only 'read' rights.
What could have happened is that you 'erased' a self created synth patch by deleting it or not saving it. Then nothing can be done, just create it again.
Did you reinitialize the 5000 to factory defaults? I believe they're still there just not available to you with all the custom tinkering you did prior.