Settings->Manage Resources in the new window that appears:
Set first pull down menu to Brush Presets and select/tick Show Deleted Resources
If you know the (partial) name of the brush: Enter this is the search section.
If you know which bundle it is from, select that bundle name in the second pull down menu. If you do not know the bundle name you need to select them one at the time and look for the deleted brush.
Once you see the deleted brush click on it and then Undelete Resources.
Krita Document Chapter. Scroll down to the Managing Resources section to get a visual about the window I'm talking about.