Why did he lose interest overnight

Before deciding what the change means, look at what actually shifted. Did his message length change, or did the warmth go? A guy who writes one word where he used to write three paragraphs is in a different spot from one who keeps replying but stopped asking you things. The first is often busy or tired. The second is more likely to be ambivalent.

Also check the timing. A sudden stop right after you shared something personal, or after a plan got more concrete, can mean he felt a jolt of pressure. A stop with no context often means something outside the conversation took over his attention. Neither of these is a clean answer, but they tell you which story is more likely before you start writing a reply.

When you reply, the goal is to lower the cost of answering, not to get a big reaction. A message that asks one simple question, or references something specific he said earlier, is easy to respond to even on a busy day. It should be shorter than what he would expect from you at your peak, without sounding like a test.

Leave out anything that asks him to explain himself, reassure you, or commit to a plan. Don't mention the gap in conversation. If your message could be read as a complaint about the silence, it will be, and he will likely answer the complaint rather than the warmth you meant.

Once you have sent one reasonable message, the next move is mostly waiting. Give it a day or two before any follow-up, unless something specific has changed, like a plan you need to confirm. Use that time to do things that have nothing to do with his phone, because the urge to check is the thing most likely to pull you into a messy second message.

If he replies warmly, match the energy he gives you and keep going. If he replies flat or not at all after a second light attempt, stop. Two messages with no real answer is a clear enough signal, and a third one turns a simple fade into something you have to manage.