I forgot to take a 40 mg tablet of citalipram last night. should i take one right now at 9 am? when i normally take them at 11 pm

Not that important: It will likely still have some in the system. Even if all anti-depressant is totally out of the system, the depression doesn't generally come back that fast nor are withdrawal symptoms common nor that fast. It has a high safety margin. Could take today's dose a bit early and just miss the one dose, by the next day's dose, the level wouldn't be a bit different.

Citalopram has a: ...long duration of effect in the body. Once it's reached a steady state, a single missed dose shouldn't be a problem. On the other hand, too much citalopram could cause a problem, again b/c of its long duration of effect. You could set up a reminder on your smart phone or find some other way to avoid missed doses in the future, but don't worry about this one missed dose.