You purchased from Amazon not Dell. Contact Amazon to check their return policy and warranty period.* Monitors have limited warranty periods, usually 90 days. 

However since the monitor worked fine for months, it must be a setting that has changed. We are all users here that might be able to help but we need to know more about your setup--computer model and operating system.  It sounds more like an incorrect setting that changed recently.

Your problem is probably caused by Hibernation which automatically kicks in after a long sleep. Hibernation requires a brief press of the power button on the computer to wake it. It looks like a reboot and takes a little longer but all open windows will appear. It has nothing to do with the monitor. You can disable or change settings for Hibernation in Power options, Additional Settings, change power plans. OR Settings, System, Power and Sleep. More info--

https://support.microsoft.com/en-us/windows/shut-down-sleep-or-hibernate-your-pc-2941d165-7d0a-a5e8-c5ad-8c972e8e6eff#:~:text=For%20Windows%2010%2C%20select%20Start%20%2C%20and%20then%20select%20Power%20%3E,down%20or%20sign%20out%20%3E%20Hibernate.

I have an issue with this exact monitor. It's been plaguing me with issues for the last year and Dell tech support insist that all 4 of my devices (MacBook and three desktops) are all faulty when they work perfectly with 5 other monitors (not dell/Alienware to be fair). 

I have to keep unplugging waiting a few minutes and plugging it in to restart it. After 2-3 minutes it just goes black. Hibernation disabled, power save mode off, cables all checked with 3 spare DP cables (perfect with other monitors) ports all checked, graphics drivers all updated on all 4 devices, but still...this AW3420DW are the issue. 

Seems to have been a while since you raised the issue, did you manage to get it resolved?

Have you tried installing the Alienware specific drivers for this monitor?  Not the Nvidia GPU drivers. The drivers for the monitor here: 

https://www.dell.com/support/home/en-us/drivers/driversdetails?driverid=fhj66&oscode=wt64a&productco...

So I have been battling this issue and I finally figured out the problem, hopefully other threads can follow my solution.

I noticed that in the gpu the power cables that connect towards it from psu, the 6+2 6+2 , sometimes the 2 might come out or is not properly plugged in all the  way, and when you game and your GPU is hitting max frames and hitting 95% GPU and it requires the power, the monitor goes black and you can still hear discord talking and whatnot.

Just take them all out and properly plug in all cables, especially the +2 cables that are separate from the +6 power cables and you should no longer have this problem, plug them in first perhaps until you hear the click sound is what I did.

Also dropping from performance mode to balanced or power saver helps but try the solution above first IMO, If this works for you spread the word in other threads please! And a foot pic in a pm as payment too LOL I joke XD