Use Rename option in Tortoise SVN from popup menu. See details here (“Moving files and folders” chapter).
When you changed the case of several file names, in other words you changed file names – they have become out of version control – that’s why SVN client noticed that files that was under version control had disappeared (apparently, that happened because SVN client wasn’t informed properly).
If you happened to be in this situation when IDE or you changed the case accidently – there is special menu option Repair move which appears when you select both files that are shown as “missing” and “non-versioned”. Check this.