Not necessarily. It could actually be his sister's birthday. That's not entirely outside of the realm of possibility. Statistically speaking, you could have asked him out on the day of his sister's birth. Luckily you have 364 more opportunities this year to hang out with him. Even if he likes you, inviting you to his sister's birthday party would be weird if you've never been on at least one date together.

Did he suggest another night? If not, he's probably not interested. Could be that he's shy. I would give it one more try, and then move on. Since you work together, it would be weird if you're constantly asking him to hang out. Maybe try to get an after-work group dinner or bar outing together and invite him along. That way if he's shy, he has others around to make it seem less date-y. Then try to talk to him alone and see if he's interested in hanging out one-on-one. 

Kudos to you for having the guts to ask him out. While "it's someone's birthday party tonight" can be used as an excuse, it's more likely that he actually did have plans. If he wasn't interested, he'd probably say, "Sorry, I'm busy" and not go into details. Though if you ask him out again, and he's like, "Uh, it's my second cousin's uncle's nephew's birthday tonight," it's safe to say he's not interested.