My friend lent me her Toyota Corolla and as I put it in my driveway I saw oil drops. I later lifted the the hood and saw oil spilled inside the top of the hood and on the belt every where. It was showing empty on the oil dip stick and a pool of oil under the car on the driver's side.
If oil drips on a pulley or a belt then the rotation of those objects could sling the oil everywhere, even as high as the bottom of the hood. Most likely an oil leak would have to be above these components and a good guess would be through something like a failed valve cover gasket.
Another possibility is that it is leaking near the front of the car from (oil cooler line, for example) and the wind from driving and the radiator fan could be blowing the oil to a variety of places, but I feel this is less likely to make it to the hood in any significant quantity.
If the oil is leaking this badly then you might be able to start the car (make sure you fill it back up with oil before you start it again) and see if you can watch and see the oil dripping.
Just to be clear, don't drive it when the oil is empty otherwise you will create bigger problems then an oil leak. If you absolutely have to drive it before fixing it, then fill it will oil and check the level frequently to make sure it doesn't get to empty before your final destination.
First job is to refill the engine with oil. Check for the correct oil in your vehicles hand book or ask at your local parts counter. Check with the vehicles owner for a history of any oil leaks. Do not start or drive the vehicle without oil. Check for the oil filler cap having been replaced correctly. Check that the PCV hoses are in place and in good condition. The PCV system vents the engine and if it is not in good working order the engine crankcase will become pressurised and force oil out of seals and joints. If the oil comes into contact with rotating parts it will be flung around the engine compartment leaving the mess you describe. With oil replaced in the engine, being mindful of rotating parts, run the engine and look for a leak. Steam cleaning is the ideal way to clean up the mess, but you can use an aerosol can of brake cleaner on a cold turned off engine, together with rags or paper towels. Hot parts could ignite the brake cleaner, but it does evaporate very quickly.
I second everything jzd and Allan Osborne recommend. I would also suggest that you clean the excess oil off before re-filling. Once you've refilled the engine with oil let the car sit for a couple hours and do a visual inspection to see if oil is leaking again and if so, from where. That will help immensely with troubleshooting.
If there is no leak then Allan Osbornes suggestions make the most sense.
As far as what could have caused this, I had an oil pump fail once and oil was coated on everything. Gaskets are the probable cause but thats speculation.
Now for practical purposes - I'd suggest that you fill the engine with oil and have it towed to a mechanic if the leak is so bad that it's emptying the 5 quarts of oil in a short period of time. Repairing a seized engine is going to be very costly and you might be faced with a replacement.