It could be a corrupt font.
But most likely it's a hardware resource issue.
What operating system are they using? How fast is the processor on the computer and how much RAM does it have? How much free, clean, DEFRAGMENTED space is on the C drive?
Are the project files saved to their C drives, to a second hard drive -- or to a network?
As Steve points out, the most common cause for a crash/hang in Titler is a bad, or corrupt font. This ARTICLE goes into more detail, and offers tips on isolating a bad, or corrupt font.
There are utilities to find and repair corrupt fonts (if the repair fails, then deletion, or replacement is required), but the font does not have to be corrupt, per se, as it can just be bad for Titler, because it is poorly written/designed. In those cases, perhaps getting that font, or one that is similar, for another foundry, is the key. Finding bad fonts can be like a Sherlock Holmes novel, however.
Going back, OpenOffice's DejaVu font was one known culprit, but I have been told that Oracle has rewritten that one, and it now works fine. Still, there are thousands of bad fonts out there.