The Gospel of Mark ends with the Greek word γαρ (for/because). Just like in English, ending a sentence with for or because is unusual, although not impossible. It is documented in other ancient Greek texts, but it’s very rare. In this context, it looks like the sentence is unfinished. Most Bibles re-order the words so it’s not obvious to readers. While nobody knows for sure, it really does look like half a sentence.
If it’s not half a sentence, then it could be translated as ‘...because they were too afraid.’
The Gospel of Mark was either never completed (some say Mark was martyred before he finished it), or the ending was lost very early in the Christian period. We don’t know.
Our Bible uses older manuscripts than most Bibles do. Check us out!