Library users in Arlington County and the public schools have been unable to search their catalogues for the past week after a computer crash Thursday.

The system failure, which has hampered thousands of residents who tried to find books, DVDs or research materials, apparently occurred when a software upgrade malfunctioned and the county’s backup system was unable to capture data from the previous week, officials said. They hope to have restored the data restored by Wednesday.

“It’s not a privacy issue. Nothing has been exposed or hacked,” said Peter Golkin, Arlington library spokesman.

The library system typically processes more than 17,000 transactions (checkouts, renewals, holds) a day. Patrons and employees at Arlington’s Central Library said Tuesday afternoon that they were unable to check online to see whether books, DVDs or other material were available. They cannot put a hold on a bestseller or see whether they have moved up on the waiting list. They can’t get a new library card or close an account. They can’t extend their loan, which means they are getting automated overdue notices.

People can still use the library, but they must consult with a librarian to locate the book, and then wait while circulation staff checks them out on a separate localized computer system.

“I was annoyed because [of] an e-mail saying my videos were late even though I knew I returned them,” Janet Masters said as she prepared to check out two more DVDs. “The next day, I got an e-mail saying ignore that notice and there won’t be any overdue fees.”

In the school system, students are unable to search the electronic card catalogues. For them to borrow books, librarians have to manually write down their names, student identification numbers and book barcode numbers. When the system comes back up, librarians will have to manually key in all those records.

“For some of our schools, that’s thousands of entries,” said Charlie Makela, the school district’s supervisor of library services.

The problem started, Makela said, when a software upgrade from Innovative Interfaces of Emeryville, Calif., failed. That glitch caused the system to crash, and the county’s backup system was overwhelmed. The vendor rolled the combined county and school libraries back to the previous version of the software but found that the data were missing.

The software is processing the backed up transactions and should be running properly by Wednesday morning, said Gene Shimshock, spokesman for Innovative Interfaces. The 30-year-old company has more than 1,400 library systems as customers around the world, he said.

The Arlington crash, he said, was the first time the software upgrade caused problems. Arlington, he added, did everything right, leaving his company’s engineers to figure out why the upgrade and backup failed. “One of the staples of our software is stability,” he said. “Believe me, this has the attention of all of us.”

Golkin said library officials think they’ve recovered most, if not all, of the lost data but won’t know for sure until the system is restored. The county’s public libraries have suspended fines and fees. Due dates have been extended through Dec. 1.

Ronald Altemus, manager of central adult services, and children’s librarian Maria Gentle said they turned to online library catalogues from Fairfax County and elsewhere to determine whether Arlington stocked certain titles and to retrieve the catalogue numbers so they could look for items in the stacks.

As Altemus described the procedure, a woman walked up seeking help locating Katherine Boo’s “Behind the Beautiful Forevers,” which just won the National Book Award. She could not find it in the library. The school library systems came back up temporarily. But Makela, who was on three conference calls with the vendor Tuesday, said her staff found inconsistencies, so the systems are not yet in use. Schools are out for Thanksgiving break after Wednesday, meaning officials still have a few more days to repair the system.