Anal advice

I don’t usually like discussing my sex life, however my husband and I tried something that we swore would never happen and I’m in need of advice. Last night we were in the middle of having sex. It was AMAZING and out of no where, he starts playing with my butthole using his fingers. One thing led to another and then he tries putting his penis in. Needless to say, nothing happened, like it would not go in. At all. So, in case this is attempted again, what do we need to do??