Steam is back up and running normally, according to Valve. Yesterday the Steam Store was experiencing what it called a "caching issue" that allowed users to see some account information from other, random users.
On its discussions page, Valve said the problem only affected some users, and the window of time during which the problems were encountered was limited to just an hour.
Valve said it doesn't believe any unauthorized actions "beyond the viewing of cached page information" happened during the window of time in which the problems occurred.
The company also made it clear that the Steam Store wasn't hacked, and credit card information and phone numbers were "censored and not visible to users, as required by law."
Users began to report unusual happenings in the Steam Store yesterday. People were seeing the names and addresses of other users, and in some cases, people were logging into the store only to have the page return a result from an entirely different region, such as Hungary or Russia.
Information that was accessible without authorization was apparently limited only to names and addresses. Of course, Steam sees a fair amount of traffic this time each year due to the annual Steam Winter Sale, which is currently in progress.
If you believe your account may have been compromised, you can try your luck reaching out to Steam Support. Unfortunately Steam has a bad reputation for customer service, something the company said it needs to work on.