Ex boyfriend advice

My ex and I were together for 3 and a half years and just out of the blue last week he left me. Everything was perfect up to that point. I haven’t spoke to him since then. I want him back but I don’t know how to get him back because I don’t even know why he left me. I have an 8 year old daughter who also misses him like crazy and he has a 7 year old son who I miss like crazy. What do I do?