SACD Player Won't Play Hybrid SACDs

A few years ago I purchased a used Sony SCD-C555ES SACD changer. I bought it to use essentially as a CD player run through an external DAC. I have about 500 redbook CDs. A couple of days ago I bought my first SACDs and began playing them. Everything worked fine until tonight. For some reason, the same SACD hybrids that played fine several times will not now play in SACD mode. I get the message "Not Hybrid" and the player defaults to CD mode. I toggle between "Multi and 2 channel" and I get the message "Not SACD." Any ideas?