There are two main reasons Platform Racing 2 can go down.
When a loader security error occurs, you will be notified when the loading bubbles show up, when instead of the normal process, it will just say "loader security error", normally on the third line. Normally, refreshing the page will work, but if not, try playing Platform Racing 2 on a different website.
A server error will kick you off of the server you're on. This error is caused when there is a problem with the server, or if the server is undergoing a DDoS attack. More often than not, a server error occurs only on one server. If a server goes down, there are ways to remedy the problem. You can try going to another server to play, although you will not be notified when the old server is back up. Every time a server goes down, that server will receive one free Happy Hour upon returning.