The last time this happened, was about a year and a half ago, over Christmas and New Year. We'd had a shocking year, he was grieving for his Mum and (as he later admitted) he was consciously pushing me away. I'm a stubborn sort, I wasn't having any of it. One night we had a huge argument. I'd been drinking (not my finest hour), and we were niggling at eachother over stupid things until he stormed out to his Dad's. Again, no contact the next day. I was phoning and texting him and his sister too (she was staying with their Dad at the time), but there was no response from any of them.
Eventually in the evening he texts to say he's fine, he'll be back in a day or two. I phoned him immediately, crying, asked him wtf was going on and could he just come back to sort things out. He was very evasive on the phone, I pushed and pushed and eventually he told me he and his sister were driving down to Manchester to stay at her flat (we live in Scotland). I was furious, hung up, and did what every mature woman does - I phoned my Mum. DP's phone was suddenly off, so I gave her his sister's mobile number - no one answered that, so Mum left a very stern message saying he either got his arse on the next train home or they'd be coming down with a van to collect me, DS and our stuff. He did come home that night, but was very annoyed with me for over reacting, said he'd just wanted space. I said what about me, when do I get peace and space? Also said I could never have done a bunk like that and left DS.
(I also discovered that one night when he'd told me he was on a night visiting a (male) friend, he'd stopped in to meet up with his sister's friend who had made it very clear she wanted him. They'd been talking a lot about emotional things that he should have been talking to me about, but chose to share with her instead. When I found out (saw a text exchange between them on his phone) I told him he'd betrayed me, and I didn't know how I felt about continuing our relationship. He was apologetic, contrite, admitted he'd crossed a line, promised he wouldn't speak to her or meet with her again).
It was Christmas a few weeks later. We spent a very fraught few days with his father and sister. We argued, there was silent treatment over Boxing Day, it was horrible. The night we left to go back to our place I overheard FIL and SIL talking about me, saying I was a terrible guest and did nothing for DP or DS, and DP could do better. I said nothing, we went home and the next day I travelled as planned with DS to see in the New Year with my parents.
I couldn't keep it secret, though, and it all came out. I phoned DP and told him everything I'd overheard, and whilst he was sorry he was also a bit "well, what do you want me to do about it?" I asked him if he felt I was worth defending, he said I was, but he was in an "awkward" position. I told him I wasn't coming home until he acted like a man and stood up for the mother of his child.
We didn't speak for a week, and I was mentally and emotionally prepared to collect the rest of my things and move back to where I grew up, get on the housing list etc. Eventually he texts me, beginning me to come back, "the house isn't the same without you, it feels lonely". I asked him to meet me halfway so we could talk. He did this, we talked, I told him how abandoned I felt and he promised to never do this again.
And yet here we are, 18 months later.
Have a 
if you made it through this post.