Are cantrips in Baldur's Gate 3 unlimited or not?

I thought cantrips could be used an unlimited number of times, but during battle they ran out. My wizard doesn't have any spells to use anymore in long battles and becomes useless.

What's the difference then between cantrips and prepared spells?

Cantrips do not run out in Baldur's Gate 3.

Keeping in mind the game is still in early access, there are a lot of bugs and glitches still being worked out. If you're saying it run out because it disappeared from the quick bar, you can re-add by dragging it from the party's spells menu (k). If it's on your quick bar but it says you need a spell slot when you click it, then try reloading a save. Either way, I recommend submitting a bug report to Larian.