this occurs whenever your memory starts running out of room, and the game has to start using 'virtual memory' on your hard drive. since your hard drive is really slow (compared to your memory), and other things in the game, such as graphics take priority over dialog files, the game just skips over the dialog.
as MiraDona posted, the easy way(and cheap) is to just restart your computer.
if its a continuous problem, you might want to invest in upgrading the amount of RAM on your computer, which is one of the cheapest upgrades you can get for your computer.
See the struggle of the faithless lot as they negate their time
How low to sink to the depths of their frame of mind