Without the error codes, any troubleshooting advice is purely guesswork. However, here are a couple of failure modes to think about:
If the READY indicator does not come on. It could be the traction (high-voltage) battery. Short of a collision, this is about the worse thing that could happen. Last summer, the traction battery failed on our 2002 Prius; and the replacement would have cost us $3,200-$3,600 (from the Toyota dealer). However, Toyota and our dealer absorbed part of the cost as a goodwill gesture for us buying 3 Toyotas from them; and our out-of-pocket expense was only $850 (which is still a lot of money).
Another common failed part is the hybrid coolant pump. It's powered by an electric motor; and when the motor fails, the pump stops and the hybrid electronics overheat. This also causes similar failure symptoms. However, after everything cools down overnight, the READY indicator will usually reappear; and the ICE will start, but for just a short period. Over the years, this part has failed twice on our Prius. Parts and labor to replace this pump is around $350-$500, if my memory is correct.
And, the overfilled oil crankcase... Over the years, this has happened twice on our Prius; but both times were when first starting the vehicle in very cold weather. After oil changes, I now check the oil level before leaving the dealer's service department; and I have had them remove oil on several occasions. While this is a possible cause, the symptoms don't quite match up; and it's not the most likely cause. However, it is the cheapest to fix.
You have not stated how many miles your Prius has on the odometer, nor if any maintenance or repairs were done recently (in the last few days or weeks). This info might provide some insight on what might be wrong.
I perform preventative maintenance and minor repairs on our other vehicles. However, the hybrid systems in a Prius (even our 2002) are much more complex than regular cars. Consequently, even though I have some mechanical experience, all repairs on our Prius have been performed by the dealer.
If you need this vehicle immediately, I suggest you have it towed to your dealer for troubleshooting and repair. From experience, you may ultimately be unable to determine exactly what has failed; and your Prius is probably going to end up at the dealer anyway. Besides, you will most likely have to purchase any hybrid system or ICE repair parts from them; because these are not available from most auto parts stores.
In 12 years of ownership, our 2002 Prius has only had about $1,600 in actual repairs (traction battery and two hybrid coolant pumps), which is exceptional for any vehicle. So, despite the inconvenience and immediate expense, the repairs you now face may not be as bad as you think (or they could be) when considered in perspective.
Note: This estimate does NOT include fuel, tires, shocks and struts, and routine/preventative maintenance.