The filter pane in a modern SharePoint Online document library is one of the more useful ways to slice a large list down to what you actually need, but it is also one of those features that occasionally just stops showing up, or shows column options that do not seem to match what is actually in the library. Here is how to get it working again.
Turning the filter pane on
In the library, select Filters near the top right of the file list, above the column headers. If you do not see a Filters option at all, check that you are in the standard list view rather than a custom formatted or grouped view, since heavily customized views sometimes hide the default filter controls.
If the filter pane opens but a column will not filter correctly
This usually comes down to the column's data type. Choice and person columns filter cleanly by default, but a column built from calculated values, or a text column with wildly inconsistent formatting across rows, often will not group into clean filter options. Check the column's settings to confirm its type matches what you expect, and consider switching a free text column to a proper Choice column if you are filtering on it regularly.
If filtering is slow or times out on a large library
SharePoint Online libraries with tens of thousands of items can hit view thresholds that make filtering sluggish or incomplete. Indexing the columns you filter on most often, under Library settings then Indexed columns, is the most reliable fix, since it lets SharePoint filter without scanning the entire library every time.
If your document libraries have grown into something that is genuinely hard to navigate even with filtering, that is usually a sign the underlying structure needs a second look rather than another workaround. Our Microsoft 365 and Modern Work team can help you rework it properly. Get in touch if that is where you are at.