Computer screen stays blank after start-up

Hello,
Whenever I turn on my PC ,the window starts,but the screen where I'm supposed to see my account (and then input my password) stays blank as like it's about to come on but it just stays like that... The keyboard and mouse will still be active and the mouse pointer still shows on the screen and it still shuts down properly when I push the power button on the CPU. Until yesterday it has never happened before. Please I'm completely lost on what to do. Please what can I do