There could be many reasons you might see that. Often it relates to something you've enabled or set that you didn't remember or which was unintentional. This could range from unintentional selections to masks to Opacity settings to even bugs in display drivers.
It's most helpful when something isn't working as expected to be very specific, and ideally to show one or several screenshots illustrating the problem. Save them as JPEGs and attach them to a post here via the little camera icon above the editor pane.
Usually there's something others can spot.
Whenever something begins acting weird "suddenly", it's a signal to trash/delete/re-set Photoshop's preferences.
To re-create the preferences files for Photoshop, start the application while holding down Ctrl+Alt+Shift (Windows) or Command+Option+Shift (Mac OS). Then, click Yes to the message, "Delete the Adobe Photoshop Settings file?"