There are a few things that could cause this.
1: Inability to connect to the database.
2. Inability to load configuration information from said database (e.g. the table has become corrupt).
3: Short open tags are disabled.
If everything was working fine before and it just started happening suddenly and you haven't made any changes, I'm betting it's either a corrupted table, or your host has made changes to the PHP configuration and disaabled short open tags.