Gosh I can't believe some of the ott negative conclusions some people have come to!
Ohhh OP, I can imagine you're extremely worried and upset, and you're playing out a thousand scenarios in your head over this, but just try to stay calm and tell yourself that the likely outcome is that he's lost his phone and you'll be able to get in contact tomorrow and all will be fine... Although he might owe you a bunch of flowers for the stress!
I have a very laid back partner and he's drove me insane in the past with making me worry. Because he doesn't panic if he doesn't hear from me it doesn't cross his mind that I could be going out of my mind with worry after trying to get hold of him all day! We've been together 13 years and his "laidbackness" still drives me nuts sometimes! A few years ago he had a Xmas party in Spain, I spoke to him in the morning and he said he'd call later that evening. So I waited for his call and nothing, so I assumed he was obviously busy so I tried calling him at midnight to say I was going to bed, and his phone went straight to voicemail. I went to bed but was a bit worried as he did say he'd call but shrugged it off at this point. Anyway, next day still nothing, so tried him again at midday... nothing... tried again later on.... nothing.... it got to the evening the next day and I was really worried by now, it had been 36 hours with no contact from him, and he KNEW I'd be wondering what was going on. I tried facebook, email, etc and eventually resorted to contacting his bosses wife to get his number, colleagues he was with, and was trying to hunt down the hotel they were at. I really was picturing the worst. Then.... he finally called very late that evening, while I sat there in floods of tears, basically planning his funeral! And although he was apologetic, he couldn't understand why I was worried?!?! Turns out they'd got so so drunk, he ran out of battery and passed out before charging his phone, then slept for 24 hours. What a state. One of the lads missed their flight back as he was still so hungover the next day!
I think when someone is that laid back, they cant comprehend what it's like to not be as chilled out as them. I bet your partner will email tomorrow and apologise for the lack of contact, and not even think for a second that he could have really worried you. Try to get some sleep and update us when you can, he's got a whole host of mumsnetters worrying about him now!!