Monitor (S2240L) goes immediately power saving mode after I turning on

Problem 1: Do have a S2240L. Been using with a cheap HDMI cable for  few months. Stayed ok. Now while I bought another HDMI cable with pretty standard quality, the monitor is showing-' Entering Power Saving Mode' as soon as I give that power. I checked all the connection. Seemed fine. I used to get 5 mins before it went power saving mode but now I'm completely clueless. What's the deal

Problem 2: Sometimes, when I reboot the machine, the CPU gets powered on but the display remain blank. nothing happens. It's happening frequently. So, I turn the main power off from the switch board. I AM REALLY SCARED BECAUSE IT MAY HAMPER MY WHOLE SYSTEM. 

Please help me out ASAP.

OS- Win7 64 bit

R7 250

ASUS M5A97 R2.0