The first thing you need to do is locate the water supply valve on the toilet and switch if to the off position. It is very important that you don't try to flush the toilet again. For water that is already on the ground, soak it up with several dry towels. It is time, temporary solution. After that Your septic tank should be pumped out by a professional as a permanent solution.

Linkback: https://www.plumbers.nz/q-and-a-toilets-wc-bidets/62/toilet-overflows-solution/1807/

Check the vent to make sure nothing has made a blockage there. If it's not covered on top, birds and critters can get in there and build nests and then your plumbing won't drain right.

If the vent is clear then you have a blockage somewhere downstream from your fixtures. Might be a ways down, so you may need to get someone to come snake it out for you. Could be roots or anything, and it may need to be scraped out professionally.