Katie Kenyon, 33, has not been seen by her worried family since last Friday morning, Lancashire Police said.
The force said a man, aged 50, from Burnley has been arrested in connection with their investigation and is currently helping police with inquiries.
Police say at about 9.30am on Friday a woman matching Ms Kenyon’s description left an address on Todmorden Road, Burnley, with a man and travelled in a silver Ford Transit, registration MT57 FLC, to the Bolton by Bowland area of north Lancashire.
She has not been seen since.
Her sister, Sarah Kenyon, wrote on Facebook: “It is now a waiting game, we have done everything we can, searched everywhere, contacted everyone."
Lancashire Police have appealed for information or dashcam footage for anyone who was in the area of Bolton by Bowland or has seen Ms Kenyon or the transit van.
Katie Kenyon
Lancashire Police
Detective Chief Inspector Al Davies, of the Force Major Investigation Team, said: “Katie’s disappearance is out of character and her family are understandably very worried.
“We are doing all we can to try and find her and I would appeal to anyone who has any information, however insignificant you may think it is, to come forward and contact us.
“I would especially like to hear from anyone who was in Bolton by Bowland or the surrounding area and saw anything suspicious to get in touch.
“Did you see a woman matching Katie’s description, either alone or with a man?
“Did you see the silver transit van or might you have dashcam footage which shows it.
“While we have arrested someone in connection with our investigation, I am very much keeping an open mind as to what has happened and I have a team of officers and staff working hard to try and find Katie and return her to her family.
“You could help with that so please do come forward and speak to us as your information could prove crucial.”
Ms Kenyon is described as white, 33, 5ft 9in, with shoulder length blond hair.
She was last seen wearing a blue cardigan, light blue and white top, black leggings and flip flops.
Anyone with information should call police on 101 quoting log 800 of April 24.
Or you can contact them through the Major Incident Public Portal at https://mipp.police.uk/operation/0401020119P01-PO1.
Alternatively contact independent charity Crimestoppers on 0800 555 111.