Now if a cat genuinely cannot pass any urine (due to a blockage), then is an emergency situation. That said, it is quite uncommon to see a full blockage in a female cat (since they have a wider urethra then the males and it would take a lot of urinary crystals/etc. to block that urine passageway). But if we have severe swelling of the urethra or a mass growing in the bladder, then obstruction could be possible for a cat of any sex. Therefore, if she is truly producing no urine (rather then dribbling bits or passing small volumes), then she should see a vet right now (even if its an ER/out of hours one).
If you double check this and find that she is producing small volumes of urine, is restless, straining, and licking her genitals (another sign that it is uncomfortable and irritating her), then our most common issue that will manifest this way in a female cat is a urinary tract infection (UTI).
If you have ever had cystitis, you will know that it is not a pleasant infection to have. And this can cause her to show these signs of discomfort. As well, we often see affected animals straining to urinate frequently while only producing small amounts of urine (often blood stained) or none at all. This is because the infection causes irritation and inflammation of the bladder and this irritation makes the bladder feel tense and sore.
To give some relief at the moment, you can try to get her to drink more if possible (which can be hard to get a cat to do). You can also offer her low-sodium chicken broth to encourage drinking. As well, aloe vera berry juice (available at health food stores) can be helpful in changing the urine pH and making the bladder more comfortable.
As well, I usually advise upping their wet food portion of her diet, as this is 35% water and will sneak some water into her system. But this will be hard if she is vomiting. And if we are seeing vomiting, it is telling us that we need to either get on top of this quick or she needs to see her vet.
While urine dilution will help flush out bacteria, it is unlikely that this alone will clear the more severe bladder infections. Therefore, I would advise that it is prudent for her to see her vet. If she can pass some urine, then it won't be an emergency in the traditional sense of the word (like a blockage would be), but it will be very uncomfortable for her and best to have her seen urgently.
If possible, then it would be wise to collect a urine sample (that can be a challenge since she is only producing small volumes). The vet will be able to analyze the sample, determine if there are bacteria and white blood cells present (signs of infection), and rule out other issues like crystals or glucose (a marker of diabetes)
Overall, we need to tread with care with Simba. IF she can pass no urine at all, then this is an emergency and waiting until tomorrow would not be advisable. Tha said, if you check and see that she has passed some urine, then I would advise trying to increase her hydration to flush her bladder out until her vet can examine her +/- dispense a broad spectrum antibiotic and kitty safe pain relief (since OTC human ones are poisonous to cats) to make her comfortable and clear any lurking infection.
Finally, if you double check and find she absolutely is not passing urine, then I would note that in our country most veterinary practices here do have contingency plans for emergency care for their patients even when they are not open. Therefore, it is worth ringing the practice. Even if they are closed, then they will likely have a message to direct you on how to contact their out of hours service. And if you don't have a vet you can find one local to you, you can check the RCVS Register (HERE) to find your local vets or Vets Now (LINK) who are open all nights/weekends. In any case, if she is passing no urine and you wanted to get her checked out sooner then there are options to have her seen now.
Please take care & my thoughts are with Simba,