Hi! Let me help you.
Start by opening Settings, going to Bluetooth & devices, and making sure Bluetooth is actually turned on. Then turn your mouse off and back on, holding the pairing button underneath until its light blinks fast so it's ready to connect. If the mouse already shows up in your device list but isn't working, click it and remove it first, then try adding it again fresh through "Add device." If it still won't show up, press Windows key + R, type services.msc, find Bluetooth Support Service, and restart it. If none of that works, open Device Manager, find your Bluetooth adapter under the Bluetooth section, and try updating its driver, or uninstall it and restart your PC so Windows reinstalls it automatically. Please let me know where it gets stuck, and we'll go from there.