Ronaldo beat goalkeeper Victor Valdes with his right-footed chip but it missed wide of the upper corner of the right goal post.
"I've scored 38 goals this season and it doesn't matter if I miss one or two," Ronaldo said. "I just try to do my best."
The English leader failed to create another decent scoring chance and was lucky to leave with its hopes intact.
Barcelona, looking for its second European Cup title in three seasons, controlled the play after the penalty but Lionel Messi and Samuel Eto'o couldn't overcome the defense.
Man United, which has managed only one victory in 17 trips to Spain, will have history working against it for the second leg next Tuesday. The Red Devils have won only two of nine semifinals. Both times, however, in 1968 and 1999, they went on to lift the trophy.
Coach Alex Ferguson is confident Ronaldo will recover.
"In Cristiano Ronaldo, I thought we had a player who could win the match. He was a constant threat and I thought that, with better support, we could have won," the Scot said.
Ronaldo vowed to add to his European-leading tally at Old Trafford.
"I didn't score no problem," the Portugal winger said. "Now I'm going to score in Manchester."
Ronaldo was United's one consistent threat and could have had another penalty in the 29th but referee Massimo Busacca waved play on. The Portugal winger had intercepted Andres Iniesta's cross-field pass to move into the area but was knocked over by Barcelona defender Rafael Marquez.
"I think he touched me, but the referee didn't give a penalty," Ronaldo said. "I don't know why. I don't understand."
Wayne Rooney played out of position and out of character while Carlos Tevez was silenced as Barcelona controlled play.
"I'm not disappointed. We played a big game," Eto'o said. "The shame was just not scoring. If we play the same way we have possibilities of getting to the final.
"We know the rival isn't easy but we're relaxed."
Eto'o put a shot into the side netting in the 53rd after Iniesta had back-heeled Messi's pass to him on the right. Iniesta had a penalty appeal waved off in the 71st.
United goalkeeper Edwin van der Sar got his hands up in the 83rd to deny substitute Thierry Henry's hard shot from the left, and the Netherlands goalkeeper stopped Henry's curling free kick two minutes from the end.
Portugal midfielder Deco took time to settle and missed a chance in the 11th, giving Barcelona coach Frank Rijkaard extra confidence ahead of the return leg that his full squad minus injured Ronaldinho should be available.
"It's still open and they know we are capable of scoring a goal away from home," Rijkaard said.
"It won't be easy, but we're capable, we just need to maintain our strength and organization in midfield. "It's not a great result, but we'll have to intend to score and wait to see how they do. It won't be easy for them."