I am not clear how the two are even associated at all. I think this may be a case where you are really dealing with two separate issues but have confused us by asking about both at the same time.
- Go to this support article to see Apple's directions about how to disable Lost Mode --> Use Lost Mode in Find Devices on iCloud.com - Apple Support. They differ slightly from the ones provided by azaksalmarzur28 who appears to have also blended the instructions for resolving the Apple Pay issue. I would tend to go with what Apple itself recommends.
Turn off Lost Mode on an iPhone, iPad, or Apple Watch
1. Go to icloud.com/find, then sign in to your Apple Account (if necessary).
2. In the list of devices, select the lost device.
If you already selected a device, select All Devices or the Close button to return to the list.
3. Select Lost, select Stop Lost Mode, then select Stop Lost Mode again.
You can also turn off Lost Mode by entering your passcode on the device. If you turned on Stolen Device Protection on your iPhone, you may need to use Face ID or Touch ID.
I am not clear as to what you mean by Apple Pay has "stopped". If you see an error message, we need to know exactly what it says. If it is not working, we need to know what you are trying and where it fails to perform as expected. Make sure it is set up properly. --> Set up Apple Pay - Apple Support
Sometimes simply restarting a device can clear up unexplained issues. Read this support article to learn how to restart your iPhone --> Restart your iPhone - Apple Support
