One night, all of my toilets overflowed simultaneously, and now if I turn the water back on to the toilet, the water keeps flowing even when the shut off system is tripped inside the top of the toilet. Help anyone?

 

Mainline is clogged up, you need to clean it out.

 

How do I clean out the main line? where can I find it, and what tools do I need to use? Thanks

 

You need to locate the cleanout outside the house, take the cap off, rent a drain cleaning machine and run the line, most will just call for help on this type of problem, it's your choice.

 

This sounds like more of a water supply AND mainline drain problem.
The mainline sounds like it may be plugged or atleast restricted,but the water not shutting off on ALL the toilets is a different ball park. Unlikely, but I suppose still possible, all the fill valves inside the toilets may have all failed at the same time.
Has there been ANY work done on the main water line in your area( call your local water department to find out) that may have broken loose sediment or other debris? If so some of that debris could have traveled into your fill valves and blocked the valves and is keeping them from shutting off. I would try replacing the fill valve in one of the toilets. If it shuts the water off then you know you have "stuff" in your water lines that MUST be flushed out.

If your main sewer line is restricted due to build up it will still need to be cleaned, if its not due to tree roots you might try a product called Drain Care. An enzyme cleaner that eats "stuff" inside the sewer lines. Its aviliable at most "big box" stores and home improvement centers.

 

The fill valves work correctly (to my knowledge) because I made sure that the fill valve was pulled up (which would usually stop the toilet from filling with more water) but the water kept flowing. Any suggestions? Also, no work has been done that would knock sediment loose (so says the water company) Thanks