The online telephone service Skype was not working for much of the day on Thursday, leaving its 220 million users, some of them small businesses that had given up their landlines, without a way to call colleagues, customers and friends.

Executives at Skype, a division of eBay that is based in Luxembourg, said its engineers worked throughout the day to bring the service back online. But they said that while they had pinpointed the source of the problem, they still did not know why it had resulted in a network failure, and they could not ensure that the service would be running smoothly again by Friday.

“There is a chance this could go on beyond tomorrow, but it’s our hope that it’s going to be resolved,” Kurt Sauer, Skype’s chief security officer, said. “What happened today was caused by a unique set of events, the genesis of which is not entirely understood.”