I left a friend on read because I didn’t want to talk to them at the moment and I didn’t know what to say, how do I explain it?

I left a friend on read and now they're questioning me and I don't know what to say. Not saying anything is making it worse and I don't know how to explain it to them. I'm really worried because this is a close friendship that I've kept close for 4 years. I'm worried I'll lose them.