On boot up, Rainbow Screen was viewed, but after that, below black screen (with 4 raspberry pi logo) was shown.... I tried various commands to open 'raspi-config' to get GUI but didn't succeed,
Only the Red LED is continuously On.
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According to https://elinux.org/R-Pi_Troubleshooting this could be several issues:
Bad SD card, either the creation of the SD card was unsuccessful or it is a bad card.
Bad power supply, USB cable...
Check trough the troubleshooting page and check your system.