Microsoft Corp., the software giant worth an estimated $600 billion, said Thursday it is trying to determine who in the company forgot to pay a $35 bill, partially crippling its Hotmail free e-mail service.

But it's very grateful to Michael Chaney, an independent computer consultant in Tennessee who single-handedly restored the service by paying the delinquent bill.

Because of the forgotten bill, some Hotmail users were unable to access their accounts over the Christmas weekend, said Microsoft spokeswoman Kathy Gill.

Hotmail has more than 52 million users around the world, but Microsoft said it was unclear how many had experienced problems.

The company is still trying to figure out who dropped the ball on the $35 payment.

"It may have been that the person who usually handles that was out of the office," Gill said.

On Dec. 24, Chaney--an Antioch, Tenn.-based programmer who works with the Linux operating system, an upstart competitor to Microsoft's Windows platform--dialed up his little-used Hotmail account. He couldn't get in and noted that the password system, which uses information found through the World Wide Web address www.passport.com, wasn't working.

"I didn't think much of it," Chaney, 31, said in an interview Thursday, noting that Hotmail had previous access problems because of technical glitches.

Hotmail still wasn't working Christmas Day. So Chaney surfed to a technology discussion site, Slashdot, where he learned why.

Microsoft somehow forgot to pay the $35 fee to retain the rights to the passport.com Web address. So Network Solutions Inc., which maintains the Web's domain naming system, essentially removed the name from the Web, leaving computer users without an electronic pathway to Microsoft's computer servers.

Chaney decided to resolve it himself. He went to Network Solutions' site, called up the data on www.passport.com--and paid the outstanding $35 with his MasterCard.

"The people on Slashdot figured it out," Chaney said. "I just took the last step." Network Solutions restored the address, and Hotmail was back in business.

"We obviously owe Mr. Chaney a huge thanks," Gill said. "We had someone here give him a call to thank him, and we'll definitely reimburse him for the $35."

On his Web site, Chaney hinted his bailout may be worth more than that.

"Microsoft is under no legal obligation to repay the $35 to me, and it doesn't really matter to me if they do or not. If they do . . . I would ask that when they make out a check they consider how much revenue would have otherwise been lost had this been down for another day or two, in addition to the inconvenience to people who rely on Hotmail as their only source of e-mail contact," he wrote.