Facebook's ad manager platform was not working for a critical period on Tuesday, leaving brands unable to place key Black Friday and Cyber Monday ads.

Both Facebook and Instagram were down on Tuesday for users. But the problems also affected media buyers, who were trying to buy ads for their clients. Buyers were unable to see data about how ads were performing, edit live ad campaigns, or start new ad campaigns. Some people had issues for an hour, while others said they had been dealing with issues for seven or eight hours. The problems affected ads running on any of Facebook's apps, including Facebook, Instagram and Messenger.

"We're aware that some people are currently having trouble accessing Ads Manager," a Facebook spokesperson told CNBC in a statement. "We're working to resolve the issue as soon as possible."

Current ad campaigns that are live and running should not be affected, but brands trying to create new campaigns or tweak existing ones may have difficulties, a Facebook spokesperson added.

Twitter

One media buyer said they've never seen errors this bad on the Facebook ad platform before. They said if ads can't be placed, it could affect sales, because customers won't know about the deals.

Another buyer said the timing was "unfortunate," and it could leave all Facebook buyers scrambling with less than a day to prepare, launch and manage ads that they were supposed to handle this week. It's also possible that some small businesses will miss the window to create new ads, the buyer added.

Both media buyers sent CNBC screenshots showing the errors on Facebook's ad platform.

Black Friday and Cyber Monday are traditionally the biggest shopping days in the U.S. Last Cyber Monday was the largest online shopping day in U.S. history, accounting for $6.59 billion in sales, according to Adobe Insights.

— Additional reporting by Jillian D'Onfro.