I am not sure what you mean exactly. Do you mean that the computer freezes or the site goes down? If that happens, you can lose credit for solving the puzzle, but you can log back in, click on the "Recent Games" tabs, and check on whether they have been "completed" or have a "Retry" option. You can then click on "Retry?" for any game you may have lost in the previous two hours, and resolve or complete.
It does happen occasionally that I I will hit the "play" tab twice without realizing it and will see only one puzzle, but will have had two puzzles load in quick succession, or that something funky will happen on the site. I check my "recent games" frequently (about every five puzzles) to ensure that I am getting credit for all the games I've played and completed and to be sure nothing strange/unexpected has happened.
Does that help?