Diagnose and fix the most common reasons a toilet won't flush, from a low tank to a broken flapper chain.
Remove the lid and set it flat on a towel so it can't slide off. The water should sit about 1 inch below the top of the overflow tube. If it's much lower, the tank isn't holding enough water to drive a flush — that's your problem to chase in the next steps.
Flush and watch the chain that connects the flush lever to the rubber flapper at the bottom. If the chain is broken, unhooked, or so slack it bunches under the flapper, the flapper won't open fully. Reattach it and adjust the slack to about 1/2 inch — enough to lift the flapper but not so much it tangles.
If the chain is fine but the tank keeps draining (water runs constantly or the level is always low), the flapper isn't sealing. Press it down by hand after a flush — if the running stops, the flapper is warped or coated in mineral scale. Wipe the seat clean, or replace the flapper, a $5 universal part from any hardware store.
Find the football-shaped valve on the supply line behind the toilet near the floor. Turn it counterclockwise until it stops. A half-closed valve refills the tank too slowly to flush. After opening it, let the tank fill completely and test a full flush.