People.com quoted Lohan as saying on Thursday's edition of "The Ellen DeGeneres Show" that the high-profile split "came out of nowhere."
"I had no idea what was going on. I just hadn't seen her in, like, a week. She, like, disappeared," Lohan said.
The singer-actress didn't talk about possible causes for the breakup, but emphasized rumors that infidelity was the reason are untrue.
"I don't believe in cheating on someone," Lohan said.
DeGeneres told Lohan that after her own public breakup with actress Anne Heche several years ago, "I kind of lost my career for a little while and I went away ... and I literally hibernated and got myself back together."
People.com said Lohan followed up that statement by remarking, "I feel like I want to get my career back in shape."
Lohan seemed hopeful she and Ronson will reunite some day.
"I really care about Samantha and we'll see what happens," she said. "Maybe when we're fully in the right place. And I love her."