Apologies if you've tried this or already know any of the following:
Battery:
Check you have a working,
appropriate battery and that it's oriented correctly. I use a "Renata 394" in my SP. The + is the top while the blank - side faces the cover.
Camera:
The meter switch is located on the left side of the camera behind the camera mount. Push it up to meter. Always be sure to push it down and deactivate the meter when finished metering, otherwise you drain the battery. If you take a picture it
should automatically deactivate the meter, but mine doesn't do this so well anymore.
Leave the camera wide open (smallest f number/widest aperture), and try different ASA settings. If you're inside the house, point it at a light when you meter.
The contacts on the meter switch can become corroded. Work the switch a bit and see if it helps.