Long story short have a friend who is super sensitive and can argue with a paper bag.
Texting recently he seemed annoyed about something so I asked if id annoyed him. he said yes id done something that had got his back up but he didnt know what. (for context - whatever id done was within this text conversation that took place over 15 minutes so I highly doubt he would have forgotten that quickly).
Same evening I sent a nice email to him telling him thank you for cheering me up so much that day - i'd been feeling down in the morning and felt like I wanted to lock myself away which I didnt thanks to him.
this set him off further apparently and he brought up how pissed off he was over the texts - with still no idea what irritated him but he knew he "got his back up" over something. 
So now were on day 2 of him not speaking to me until I apologise.
i'm at a loss to what im apologising for, but have said sorry he took whatever ive said to heart but without telling me what ive done i'm at a loss to know what i'm apologising for.
I guess i'm asking should I just apologise when I feel he's being a Huge, massive crank right now and needs to settle himself down out of his mood swing. I will always apologise if im in the wrong but on this occasion all over done was check he was ok and send him an email thanking him for being so good and kind to me.