Had a record day yesterday and then today 0 sales so far. Why is this happening?

Inconsistencies are ok. Lower the spend and test gradually. Once you see the ROAS is coming back, increase the spend back. That’s in case your metrics: CTR link, CPC link are according to KPi’s and competitive: Facebook Ads KPIs Cheatsheet. If not, then could be your ads are just weak and then inconsistency will be a constant for you