My daughter has told her teacher I slapped her.

[Blaaaaaaaaah]

I don’t know what to do or how to explain to my five year old that lies like that have real consequences. I’m so worried now.
I accidentally scratched her with my nail whilst attempting to brush her hair, she was mid tantrum which is why it was so difficult. She was fine when she went to school but did have a red mark down her face. She says the teacher asked her why so she said I’d slapped her, I’ve asked her why but she says she didn’t know what else to say.
What do I do now? And what do I say to her?

First of all, don't panic! It's easier said than done I know. Have a word with the teacher and explain! Say what you said here.

It sounds like she just muddled slap and scratch, which is what the mark was. Explain to the teacher, it'll be fine OP

Teacher will know it's not a slap mark because it's a scratch. They'll have logged it on their child protection software and someone with safeguarding responsibility will have either spoken to your DD about it already to establish more detail or will be planning to do so on Monday. They may ask you about it. When your DD gives more detail it will be obvious that it was an accidental scratch rather than a deliberate slap and they'll consider the matter closed. If the school is particularly officious, they may report it to social services, who will then either speak to you/your kid and close the case (if you live somewhere where there's a small case load) or bounce it straight back to the school to investigate further and only contact them again if they've got more evidence than that (likely if you live in a city, especially a poorer city).

I wouldn't bring it up TBH. If they consider it worth investigating, they'll ask you about it and you can tell them then- this sort of thing happens all the time.

Its ok, just have good communication with the teacher. Children often experiment with the thrill of lying, and then learn that it upsets people. It can be connected with impulsivity and low self esteem if its a longer term problem. The other thing that can happen is that sensory defensive children can misinterpret normal touch as 'hitting'. If its a one off five year old miscommunication just laugh it off. If she has any homelife issues, address them and spend extra time on helping her feel important and noticed. If she struggles with normal sensory experiences, consider learning more about her sensory needs or getting a sensory profile assessment.

My nephew when aged about 5 always told school he wanted to smoke /that his parents let him smoke / that they spent the evening smoking etc.
Parents have never smoked. Ever. They've no idea where or why he got the idea he wanted to smoke.
But it was simply cleared up with a quiet word, teacher suspected was not from a smoking family as his clothes simply didn't smell of residual cigarette smokers.
Now, as a teenager nephew is very anti smoking and would never dream of smoking/can't explain his early behaviour.

Children make disclosures like this 50 times a day!

They're often not lying, exactly, just seeing the world from their perspective.