The Playstation Store will be receiving a brand new interface in the coming weeks, starting with Europe on Tuesday, and North America following on October 23. To prepare for this new update to the store, there will be some maintenance this Monday. It will take place from 4AM to 4PM PST, and will mainly focus on the store and account management side of the network. While you may still have problems signing in during the downtime, this type of maintenance usually will still allow you to play online. You should only be blocked out of using the Playstation Store and Account Management.
Source: PS Forums