I don't know why, I haven't contacted my good friends for several months, and I haven't returned any messages. Now I want to reply. What excuse should I use to say it?

It's not too late to come back. You can pretend that you didn't receive the text messages you sent in the past, send him a text message on your own initiative, ask him how he is doing, ask him why he hasn't contacted recently and forget what happened in the past. Act natural. And that means you've been busy recently. Anyway, pretend nothing happened. Because the other person may not care about your behavior at all.