Is this for your home’s water heater? I was initially under the impression this was going to be for your car.
If this is your home’s water heater, it is possible that by turning it off it did turn out. Some water heaters have a pilot light that needs to be lit and by turning it off, you extinguish that flame and it needs to be relit. It depends on each water heater in the manner how you will relight it, but most will have you either turn the knob to a certain point and hold a button while you light it using a match or they will provide another button you press which creates a spark which will hopefully light the pilot light. You need to be watching the pilot flame as you do this to see if the flame sticks. If it does, then you can move the dial to an on position to have the flame fully light up, almost like how a gas flame lights up on your gas stovetop.
All this is general but these directions in between are more direct depending on your specific make and model. The best thing you can do is to either watch a YouTube video on your specific make and model to see how it’s done or look up the manual online. Also, in most cases from the many I’ve seen, they should have these written as directions on the side of the tank. They can often be a bit hard to read and follow for the first time, so be patient and don’t be afraid to re-read sections again a few times to be sure they’re saying what you understood.
There’s no much risk involved here for the most part if you’re safe, but the taking time part is mostly because I’ve had issues pointing the dial to the correct position and it creates confusion and frustration for me. But taking time to re-read it before acting helps reduce those occurrences.