On the log in screen press and hold the SHIFT key and try to click on the power button and click on restart. If it's worked it should boot to the Advanced startup options screen. Select Troubleshoot then safe mode. Reboots to safe mode then log in. If it's working OK running in safemode it could be a graphics problem or that your screen resolution is wrong. Has the graphics driver been updated? Also right click on desktop, screen resolution and see if set to recommended settings.
Alienware 17 R4 2017 review