Q: i have an iphone 6 plus that is water damaged but i got it fixed and the person put it on charge and tested it with a multimeter and it worked perfectly but when i went to get a screen tested on it it didnt work

Check the Liquid Contact Indcator.

Instructions here >  Liquid damage to iPhone or iPod isn't covered by warranty - Apple Support

Just so you are aware, service for the liquid damage to an iPhone or iPod isn't covered by the Apple one-year limited warranty or an AppleCare Protection Plan (APP).

"got it fixed and the person"...

It would have been better to take the iPhone to an Apple Store or Apple certified reseller.

iPhones are not user serviceable and Apple does not sell iPhone parts. Now that it's been tampered with by an unauthorized party, Apple won't touch it. It is no longer eligible for a discounted out of warranty replacement.

The likelihood of being able to successfully resurrect a liquid damaged phone (and have it be safe to use, i.e. not a fire hazard) is slim.

The "fix" is easy, but you won't like it... recycle it and buy a new phone.

A 3rd party repair shop might be able to help you, but it won't be cheap. You also have to keep in mind that Apple does not sell iPhone parts at all. By physically opening up the phone, you've forfeited your right to have it serviced or replaced by Apple. They won't touch it.