I would guess (without seeing it) that you have one of the old-series 33Es that were built with minimal solder. The ICs, display pins, and flexiprint from the power converter (this module is soldered together) are clamped against a foam-backed flexible PCB by a plastic frame.
In time, the contacts oxidise or get dirty and you get faults like display digits, or segments, missing, some keys fail to work, the whole machine is dead, and so on.
You have to take it apart and clean the contacts. Dismantling a Spice (HP3xE or HP3xC) machine is not trivial until you've done a couple, and there's a lot of misinformation out there about separating the case halves. Here's how to do it :
Remove the battery cover, battery pack, and the 2 screws inside the battery compartment.
Ease the case apart at the battery pack end. Do NOT keep on separating the case here (as is often recomended) -- this is a good way to break something.
Now put your fingers in the battery compartment and your thuimbs on the front edge of the top case. Squeeze hard. The idea is to move the bottom case towards the front (0-key) end of the machine. It'll suddenly move with a sharp click.
Now carefully remove the bottom case, making sure you don't damage the battery contacts.
Free the battery terminal PCB from its mounting
Unclip the logic assembly from the hooks down each side. Take care here, if you break them off, you'll be looking for a new top case. At this point the keys, switch sliders/contacts are loose in the top case. Put it all aside so you don't scatter parts everywhere. If the switch contacts are stuck to the logic assembly, remove them and put them back in the sliders.
Remove the metal clips from the sides of the logic assembly
Now, with that assembly keyboard side up, lift off the keyboard/flexible PCB. You'll be left with a plastic frame containing the chips, the PSU module, and the display.
Clean the contacts on the flexible PCB, clean the display pins. Put it all back together.
This is a very common problem with early Spice machines