[Typo3] fatal error in frontend

[yringgenberg]

hello,
until yesterday my front end was ok and since today I have this message:

Fatal error: Call to undefined function: imagettfbbox() in 
/Volumes/data/wwwRoot/server-2/t3lib/class.t3lib_stdgraphic.php on line 
921

I havent' done nothing special, so I realy don't know what to do, the 
backend works perfectly...

Somebody has an idea?

thanks

yann