I do not have it on my computer.

Once your computer reported a task as completed, there's not really a way to do it again. If it still says "in progress" on the website you really have only 2 options. One is to let that task expire in a year when it's due. Two is abandon the task, and the only way to do it is by detaching from the project and reattaching again. The task will be reassigned to another user. You'll still keep all of the credit granted to you for it. If you decide to detach/reattach, I'd suggest to make sure that before reattaching the "last contact" date (not just the time) on the website is different otherwise you may not be able to use the merge function on the website to merge the two computer instances. When you reattach you may not need to merge as they'll be merged automatically as long as key specs don't change but I'm not sure about that.