Hello;
I have one 11C, two (new)12C, one 15C and one 16C. The message Ellis mentions is "Pr Error" (they're all 7-segment display), and it should appear when [ON]/[-] are pressed, as Thibaut mentions. What I strongly recommend is that you shorten (very briefly) the batteries contacts (use a thin wire) WITH THE BATTERIES IN. Depending on the model you have, the batteries will not fit in if placed inverted.
If the calculator does not turn ON, the alternate reset is [ON]/[D], being [D] the key with y raised to x printed in. Press simultaneously (not one after the other, both at the same time) [ON] and [D]. The display changes its contents IF the calculator turns on.
I repaired both HP12C (old-fashion) and another HP11C by simply resoldering all IC's terminals (one IC in the HP12C, two IC's in the 11C).
If you need more info, let us know.
Success!