Leaking from the bottom

When you place the tank in the unit, it allows water to pass through a hose to the water pump. It sounds like your hose is leaking or disconnected; first thing I'd do would be to open it up and check the to see if that's where it's leaking from.

Fortunately, someone else has already run into problems with their carpet cleaner and created guides to a few repairs, and luckily one of them shows how to replace that hose.

Bissell SpotClean ProHeat 5207U Hose Replacement - iFixit Repair Guide

Keep in mind you may not have to replace the hose if it's just come loose from one end, but take it out and examine it for any cracks or bulges that might be leaking. Check the connectors on both ends of the hose for any cracked or broken plastic that could be leaking. After you open it up to where you can see the parts, you could put the tank back on and at point the leak may be obvious.

Best of luck; come on back and let us know what you find - your experience may well help the next person who has a leaking carpet cleaner!