Elaborating on what sapi said:

The latest patch for R:TW is 1.5. To find out what patch you have on your R:TW main menu click on options. In the top right hand corner you will find a number.

If it is not 1.5 and is instead 1.0 or 1.2 then
  1. Download patch 1.3
  2. Download patch 1.5
If it is 1.3 then:
  1. Download patch 1.5
If you have the latest patch (1.5) post reasons as to what you were doing when the problem happened and what you think could have caused it in the Tech Support forums.