The app has disappeared on my watch, how do I get it back

Start by force restarting the Watch and paired iPhone at the same time:-

Force restart the Watch

i.e. press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo. Then while Watch is restarting,

Force Restart the paired iphone   Force restart iPhone – Apple Support (UK) 

You may also want to make sure the software on the watch and iphone  are both up to date to the same level. Updates are in pairs. iOS16.5 and watchOS 9.5

Update your iPhone or iPad - Apple Support and

Update your Apple Watch - Apple Support 

Test it for a while. If it is still misbehaving...

Unpair and set up again

 Unpair and erase your Apple Watch – Apple Support (UK) 

and then set up again - takes about 20 minutes

Set up your Apple Watch – Apple Support (UK) 

i had a app on my watch but it disappeared over night it is on my mobile but it not listed as an available app on watch can someone please hekp

This has been answered.