It is possible to make an object or avatar “invisible” - for you only. This is known as derendering. It is useful in many situations, such as photography, for example, if an object is in your view and you do not wish it in the picture.
To derender an object, right click it an select Derender from the menu. You may select temporary or permanent (blacklist) derendering. A general summary of these two types of derendering, and how to reverse them, is given here.
In order to reverse a permanent derender, you will need to open the Asset Blacklist, via the top menu bar → World → Asset Blacklist.
Do not derender or blacklist an object you are sitting on. If you do, you will suddenly find youself in a corner of the region, still sitting, and without a “Stand” button; you will have to TP or relog to free yourself, and may still be stuck in the sit pose.
The Asset Blacklist window, shown above, lists all the items you have blacklisted. The list shows the name of the item, the type (object, sound, etc), and the date added.
To remove an item from the blacklist, click on it, then click the Remove selected items from blacklist button.
At this point, you will need to force your view to redraw. This can be done by changing your active group, or by TPing out and then back.