I think my mom shut off her cellular service by accident.. how does she turn it back on.. t was her iPhone

Do you mean she cancelled the service with her carrier? If so, she needs to call them and re-instate it.

Or do you mean that she did something to the settings on her phone that is preventing her from accessing the cellular signal? If so, the most likely culprit is Airplane Mode. Settings>Airplane Mode>Off.

What happens when she turn the celluler service On again?

Try this : remove and reinsert the SIM card.

See also these Apple articles below:

If you see No Service, Searching, or SOS on your iPhone or iPad

If you see No Service, Searching, or SOS on your iPhone or iPad - Apple Support

Manually update your carrier settings on your iPhone or iPad

Manually update your carrier settings on your iPhone or iPad - Apple Support

Update your network provider settings manually on your iPhone or iPad

Update your network provider settings manually on your iPhone or iPad – Apple Support (UK)

If you see a Cellular, Ultra Wideband, or Apple Pay Issue message after an iOS update or restart

If you see a Cellular, Ultra Wideband, or Apple Pay Issue message after an iOS update or restart - Apple Support

Did she set it on airplane mode?