Guided access has locked my phone

Perform a Forced Restart on the phone. Follow the instructions very carefully, as this is a bit tricky. You may need to repeat this several times.

Force restart iPhone - Apple Support

There are only 2 possible results when you perform a Forced Restart:

1) The phone returns to normal operation

Or, if you are still locked out.....

2) There is a hardware problem with the phone that will need attention at the Apple Store (make an appointment) or take the phone to an authorized Apple Service shop.

You can find a list of authorized service shops in your area using the link just below:

Find Locations

Have you tried the suggestions here? Apologies if it's too basic a suggestion.

Use Guided Access with iPhone, iPad, and iPod touch - Apple Support (PH)

I can’t even unlock the screen so will this still work?

A Forced Restart will always work unless there is a hardware problem on the phone. Why? You are using the screen, you are using the buttons on the phone. The process terminates any apps that are running on the phone so the phone starts up again normally.

If you take the phone in to the service shop, the first thing that they will do is run a Forced Restart a few times, so it makes sense to try this.....before.....you take the phone in.