Credit: Lester Cohen/

Megan Fox is engaged again, reports the new Us Weekly (on newsstands now).

On-again beau Brian Austin Green popped the question to Fox, 24, June 1.

"Yes, she is engaged," her rep tells Us

But in all the excitement, the happy couple somehow lost the ring, sources tell Us Weekly.

"I saw her jumping up and down," a witness tells Us Weekly of the beachside proposal at the Four Seasons Hualalai hotel on the Big Island in Hawaii. "Later, I saw a half dozen staff sifting through the sand."

Alas, the 2-carat sparkler, which the pair picked out together from Excalibur jewelry store in Beverly Hills, remains MIA.

"Security and maintenance staff spent a couple of hours looking for it," another source tells Us Weekly. "No one found it."

Green, 36, and Fox -- who next stars in Jonah Hex (out June 18) -- began dating in 2004 but called off their previous two-year engagement in February 2009.

Could kids be in the future? (She is helping raise Green's 8-year-old son, Kassius, with his ex, actress Vanessa Marcil.)

"No one believes me when I talk about this, but I'm really maternal," she told W magazine earlier this year.