My HP 48 G started blinking the ((.)) sign (same as alarm). I decided to reboot it, so it didn't turn on again. I tried hard reset using the reset on hole, ON+C and even change the batteries, with no success. I didn't found anything more on user guide.
Are you sure that the batteries are in the correct orientation (with the + ends facing left, right, left from top to bottom) as indicated inside the battery compartment? Are you sure that the batteries are 1.5 volt alkalines? Have you tested their voltage outside the calculator?
If it still won't turn on, try a master reset by pressing the ON and A and F keys at the same time, then releasing them. Several attempts may be necessary before you see "Try To Recover Memory?" displayed. Press the NO key to clear memory to its original factory state and recover control of your HP 48. Some HP 48's just beep when NO is pressed; if this happens, press the E key next to the NO key, then press NO.
If none of the above helps, my last suggestion is to press and hold down the bezel between the C key and the display while attempting to turn on the calculator. If that works, the ribbon cable inside is failing.
Disclaimer: I don't work for HP. I'm just another happy HP calculator user.