Short answer
A stopped watch is not automatically “buy a 377.” First decide what kind of movement you have — quartz (replaceable cell), solar / Eco-Drive / Kinetic (usually no user coin cell), automatic or manual mechanical (winding / service), or a smartwatch / fitness tracker (built-in rechargeable).
Only after you confirm it is a quartz watch with a user-replaceable cell should you match the code on the empty cell or the Casio module path on this site.
30-second check
Second hand ticks once per second — usually quartz; a dead primary cell is a common cause when it stops.
Second hand sweeps smoothly, or the dial / case says Automatic — usually mechanical; try winding or wearing before shopping for a battery.
Case or dial says Eco-Drive, Solar, Tough Solar, or Kinetic — do not assume a jeweller “button cell” fix; follow the maker’s light / charge / service path.
Touchscreen watch or fitness tracker that charges on a cable or cradle — sealed rechargeable; there is nothing to replace with a coin cell.
If it is quartz and needs a cell
Read the code on the empty cell (CR2016, 377 / SR626SW, 371 / SR920W, and so on). Prefer silver oxide when the cell or chart names SR / 3xx trade numbers — see LR626 vs SR626SW for the alkaline lookalike trap.
Casio models: use the Casio watch battery finder (model → module → battery). Brand-alone pages that say “Casio watches use X” are not how we publish answers.
Trade number without a device page: use the watch battery chart, which lists alkaline twins separately instead of flattening them.
When to see a professional
Water-resistant, screw-back, luxury, or warranty-covered watches — opening the case yourself can void the seal or warranty. Ask for a pressure test after a battery change when the watch is meant to stay water-resistant.
If the watch still will not run after a correct fresh cell, the movement may need service — another cell will not fix it.
Quick takeaway
Diagnose the movement type first. Buy a cell only when the watch is quartz with a named replaceable code — then use the chart or Casio module path, not a brand-wide guess.