I've lost my student card on campus, as well. =/
Luckily someone found it and brought it to Thode a day or two later.
I wouldn't suggest getting a new card unless you ABSOLUTELY need it, i.e. a test (though I doubt it's likely atm, term just begun), so I'd wait a couple days. Sometimes people wait a day or two before getting around to bringing it to the registrar.
Hmm, as for putting it on hold, I'm not sure what you'd do if it's closed on weekends..
I guess you'd just have to go look for it and if you still can't find it, put it on hold on Monday. =/
Good luck, I hope everything works out.