Hi;
let me try to get more info.
Is the "5" the only one character that can be seen or it is a "5_" (five with prompt cue, or underscore)?
After switching to ON, are you able to switch the calculator OFF by pressing the [ON] key? Or you have to remove the batteries?
If the display shows "5_" and you cannot turn the unit OFF by pressing the [ON] key, chances are you have a permanent pressed [5] key (short circuited, broken metal spring, among others). Fixing the key (that's another story) will probably be enough. I once fixed one HP41 with a related problem. One small piece of plastic locked the key (I don't remember which one) and it simply did not turn OFF and insisted on showing something (what was it?).
Otherwise, if the display shows only "5" and you can turn it OFF and ON by pressing the [ON] key, I cannot see the reason. Defective IC?
Others will post suggestions, too. And I hope this helps.
Success!