My toilet worked fine yesterday. However, over night, the water in the bowl backed up and overflowed without anyone flushing the toilet. The water is clear. I don't understand what is causing this problem.

Water is not draining completely from the toilet line to the sewage tank. If there is a 'low/little slope' section anywhere in that drain line, solids WILL settle and collect there and cause water to back-up in the toilet drain line. Then the toilet will not flush completely or at all until the 'soft blockage' is cleared out and the drain line emptied. Al 'gravity drain' toilets require a free draining/empty drain line that allows flush water to drain unrestricted. A basement toilet has a pump that 'forces' flush water up and out with enough pressure to sometimes force out the 'soft clog.'

Troubleshoot Your Toilet Without a Plumber

www.popularmechanics.com/home/how-to/a4117/4303948/

Overflowing, constantly running and sweating toilets can test a home owner's sanity. ... take for granted--until we can't take a shower or make the toilet flush properly. ... If none of the above does the trick, you may have a misadjusted or faulty fill ... the waterlevel a half inch below the top of the overflow tube inside the tank." ...

Emergencies - Overflowing toilets - Toiletology 101

www.toiletology.com/emergenc.shtml

You can't just lower the water level in the tank; you need to displace some of ... and the ballcock and flapper are working properly and shut off when the tank is full. ... The bowl is filled with the water that goes into the overflow pipe. ... Unfortunately, when a toilet bowl is clogged and the water level rises and doesn't flush the ...

Is the flapper valve closing sooner then it should? Is the water line (the 1/4" vinyl tube at top of fill valve) going into the tank or into the overflow tube? Did anyone drop anything into the flush valve?

Toilet runs continuously or doesn't flush adequately

Jiggling the handle might work for a while, but if your toilet is always running or doesn't flush adequately, you need to perform these simple fixes that usually can tame a noisy, inefficient toilet.

Check the Chain

The chain attached to the rubber flapper often becomes loose, causing incomplete flushing and/or continuous running. To fix it, remove links from the chain using needle-nosed pliers until it's tight.

Check Water Level

Improper water levels can also cause problems with your toilet. This can be fixed by adjusting the ball ****, which is located next to the overflow pipe, the open tube to the right of the handle. Adjust the ball **** so the water comes to ½ " below the overflow pipe.

Clean the Surface of the Flush Valve

A continuously running toilet can also be the result of a faulty flush valve. Because the valve is made of rubber, it can decompose or warp allowing water to escape from the tank into the bowl. Clean the sealing surface to see if this will improve the seal. If too decomposed or warped the valve may need to be replaced.

Replace the Flapper

Remove old flapper and replace with a suitable replacement. Attach the flapper to the overflow pipe inside the bowl. Then attach the lever chain from the flapper to the handle lever making sure the chain is taut.

This video will help familairaize you with the flushing mechanism and the repair:

The water level in the bowl is determined by the built in trap in the back of the bowl. Water is allowed to enter the toilet and it then "over flows" the p trap in the back when flushed. Once it's flowing, it syphons the water out of the bowl. While the tank is re-filling, a small portion of water should be running down the overflow from the small hose clamped onto the overflow This rinses the inside of the bowl, as well as helps-refill the bowl. Be sure that hose is directed inside the overflow tube. To see what the max water level in the bowl can be, add some water from a cup or glass. Once it's full you can keep adding and it won't get any higher. This will be the max level you can expect.

The small black tube that goes into the overflow is used to fill the bowl after flushing. Would check to make sure that water is coming out of that tube at full force or if its coming out at all.

replace flush valve.

Lift the cover on the toilet tank. If the water is too high, it is running into the overflow tube. You will have to adjust the float lower to stop the water at a lower level. If there is no water in the tank, the ball **** is faulty and needs to be fixed or replaced.