If I lock my phone can I still use find my phone app in case it is turned on.?

Yes. Find My operates as long as the device is powered and connected to a network. It doesn't matter if the phone is locked.

In fact, the preferred action when you lose your phone is to lock it first. This prevents someone else from using the phone if they find it. Even if the phone is off when you try to lock it, it will lock as soon as it next connects to a network.

jfromnorth wrote:

Have lost phone , Find my Phone says it is not connected

if I lock that phone can I stlll keep trying with find my phone?

Read here If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

> icloud only locates my ipad, not my iphone but it is listed as a device

That likely means its turned off - either you're lucky and the battery drained, or you're unlucky and someone found the phone and turned it off.

However, Find My should also report the last known location. If you don't see that it's possible that Find My was not enabled on the phone, n which case you're out of luck.