Yamaha Ex5r problem

I have an ex5 that will only allow me access to one midi channel at a time. therefore only one sound at a time. The F1-F8 (bank select) buttons are inoperable. I love the sounds in this thing and am really reluctant about sending it to Yamaha for a problem that may not be as critical as its effects on my music.
If someone has an answer for me pleeease help me......