Is the toilet one that's pumped manually, or electric (you push a button to flush it)? If it's a manual toilet, it pulls in sea/lake water...trace the small hose coming off the back of it to a thru-hull (hole in the bottom of the boat) that has a seacock (lever operated valve) that opens and closes the thru-hull. Turning the lever to line it up parallel with that hose opens the thru-hull.
Some electric toilets also use sea water--in which case, find the thru-hull and open it...some are designed to pressurized freshwater from the boat's potable water supply. In either case, it may be as simple as just making sure the toilet's breaker is on.
The owners manual for your toilet will help you learn how it works. If you don't have one, you can download it from the mfr's website. I can post the link to it if you can tell me the make/model. If you don't know or can't find it on the toilet, post a photo that shows the pump or anything under the bowl, NOT the bowl (all bowls look alike).