Assuming you are referring to the webbing on a piece of furniture:
There are a few things you can do to fix sagging webbing. One is to remove the old webbing and replace it with new webbing. This is probably the best option, as it will give you a chance to inspect the frame of the furniture and make sure there is no damage.
Another option is to simply tighten the webbing. This can be done by removing any slack in the webbing and attaching it more securely to the frame. This may not be a permanent solution, but it can help improve the appearance of your furniture until you have time to replace the webbing.