It would help to know what kind of mower you have? A self propelled hand mower would get a completely different answer than a Zero turn.

With that if the mower is not moving correctly, it is possible there is a broken belt, or broken spring on one of the tension pulleys in the system that keep or draws the belt tight.

Good point: I will edit that and check the tension spring. To be continued............

Could be the belt came off the trans axle pulley or you forgot to push a button. Then again your mower was made by Husqvarna or MTD and came with very cheap trans axles, both will leak out the hydraulic fluid or if an MTD, just prematurely wear out.

I'll check the belts, but what button did I forget to push?

' The transmission lever in the rear is pushed in'

You mean the transmission cut-out rod? Pull it out. Maybe you bumped it somehow.