If my supplier changes something at their end, will my booking site suddenly stop working?

Usually no, but it can happen, and it is worth knowing why so you are not caught out.

Your website talks to each supplier through their API, which is simply an agreed way of asking questions and getting answers back. Suppliers keep improving those systems, and now and then they change something, add a new field, retire an old version, tighten a rule about how a passenger name or a date must be sent. Most of the time these changes are announced weeks in advance and the old version keeps running alongside the new one for a while, so nothing breaks on your side. The connection carries on quietly and your agents notice nothing.

Trouble comes from the changes nobody warned about, or a notice that went to an old contact address at your office and was never read. Then a search may return no results, or bookings go through everywhere except with that one supplier. The site itself stays up, because a properly built portal treats each supplier separately and simply drops the one that is misbehaving out of the results rather than failing altogether. Fixing it means adjusting the connection to match what the supplier now expects, which can take anything from an hour to a few days depending on how big the change is.

Make sure every supplier has a current technical contact for your business on file and that their notices reach someone who will act on them, then check your search results supplier by supplier if bookings ever go quiet.