Casio CZ/ VZ/ FZ - Pro Series

Purchased a nicely looked after CZ5000 from a friend a few months back.
 
It has gradually become slower & slower to switch on. The display shows a line of boxes & then after a while there's the click & characters/sounds appear. This  delay was initially intermittent - sometimes it would switch straight on.  Last night (isn't it always the way) about to use it for a studio project to find it wouldn't get beyond the grey box stage - no click, no characters, no sound & no buttons operational.  Have checked/tested the 3 AAA batteries which are all good. 

Is there an internal battery to check, or is this irrelevant?  I want to avoid spending at least what it is worth getting it prof repaired if possible.  Greatly appreciate any help.

Thanks

Tim