'Do not do normal boot invalid kernal length'

Hi. I've got a Samsung Galaxy A40. Today all I have on my phone is a circle in the middle with a lightning flash and in tiny red letters at the top right of the screen it says 'Do not do normal boot invalid kernal length'. The battery should be full as it was on charge last night. I cannot turn it on or off. If I press on/off and volume at the same time the Samsung logo comes on briefly and that's it.

I have no idea what to do. Obviously I Googled it and I watched a YouTube explanation of what to do involving down loading Odin but I really couldn't follow it.

Can anyone help? Please bear in mind I am almost a complete Luddite so need things explained in words of one syllable......

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