If you have zero venting on a system, the toilet will not function. The flip side of this is that a toilet does not need to be individually vented, for reasons I mentioned previously. Building on what MightAl wrote, there is a way to determine if you have a totally blocked, or non-existent vent system, as your description of events seems to indicate that the shower drain acted as a vent. Get a toilet plunger and violently plunge the shower drain. First look in the drain with a flashlight, and determine if you can see the standing water in the U-bend of the trap. Once you plunge, hopefully you blew all of this water past the trap and created a temporary vent. You might be able to get your nose down there and notice a bit of a sewer gas smell? Now you have a temporary vent system, since the drainage piping has access to outside air. Flush the toilet. Is the toilet working well? if so, you have a blocked, or non-existent venting system. If not, you may have a blockage in the drain below the toilet. Once this experiment is done, run some water in the shower to reestablish the drain seal. If you have a three piece bath with a sink, but no venting, you might be able to get it to perform adequately with a Studor valve, AKA an Air Admittance Valve, or AAV, hidden under the sink. This may not be 100% code approved, but it could make a huge difference and cost $30-40 in parts, not thousands in re-plumbing. Good luck.