Which version of Photoshop and OS? How much RAM and spare disk capacity do you have? Have you tried resetting your Photoshop preferences, which you can do under the General tab in photoshop preferences?
Hello, thank you so much for responding - I am using mac OS Catalina V.10.15.7 and I was using Photoshop CC 2021.
After finding advice on different forums and blogs I tried firstly by editing preferences in PS inc. the available RAM, graphics processor settings and caching... I then completely uninstalled CC from my computer and re installed it - nothing worked.
I have been at a loss and decided to go back to photoshop 2020, once this was installed I opened the file I was having trouble with and it works fine.. so I think I will wait a little while until I upgrade!