Should I reply to my ex who messaged me after months?

Don't respond when you're heated or at three in the morning when your head starts spinning. Take some time (a day, two days) to think about what you want, not what he's trying to get with that message. If after that time you're still not sure what's going on, it's probably better not to respond - people who matter to you don't disappear for months and then come back like nothing happened. If you do decide to answer, keep it short and don't reopen everything.