Wondering why Casio PT-1 suddenly stopped working (won’t power on).

I have had mine since originally buying it new in the 80s. It worked fine until just recently when it stopped powering up. Unit is clean in very good condition so a bit baffled why it stopped working suddenly. I do not have AC adaptor and am using batteries. The battery compartment is also clean. I expect it is a small easily fixable issue so am wondering if anyone has experience with this / similar models and may have suggestions what to check and how to fix it. Thank you in advance to anyone who may be able to help!