You were right thinking there were supposed to be open shop hours tonight. The shop never opened because no one signed up to staff. (Actually, that's not exactly right. Last Saturday no one signed up, but a few of us just showed up to make sure people could get in and we ended up staffing until the end of the shift. Anyway, apparently no one did that either tonight.)
Given that FR is entirely volunteer run (or sometimes volunteer *not* run, as you've unfortunately experienced), this could easily happen again: it might be worthwhile to call the shop before heading out, especially if it's out of your way to get there.