The best way to search for labs in the HOL portal is to first select "ALL LABS" item from the left hand catalog selector and then type in the keywords or lab SKU you are looking for, that will search for all labs across all the catalogs, else you will only be searching within the selected catalog.
That said, we do from time to time have to disable a lab for maintenance so that lab might have been disabled for maintenance when you searched last time.
Also, we are just about to remove all our old 2015-2016 content ( the labs that begin with HOL-SDC-16xx) but there are equivalent updated 2016-2017 HOL labs that start with HOL-17xx that will cover the same material.
For the HOL-SDC-1632 lab that you were interested in, please take a look at the new HOL-1721-USE-3 - vRealize Automation Advanced Extensibility Lab.
I hope that helps,