Text START (in all caps) to 57439 from the same phone you're using for your Split Pay account. This re-subscribes you and login codes will start coming through again.
Once you've sent START, try logging in again. The code should arrive within 30 seconds to a few minutes. If it doesn't, see the FAQ on I'm not getting the text with my Split Pay login code. What do I do?.
A heads-up: STOP and START work one number at a time. A STOP sent to a different Split Pay number doesn't block the login codes we send from 57439, and texting START to 57439 turns texts back on from that number only — it won't opt you back in to marketing texts you've unsubscribed from on another number.