Not able to access EC2 Linux instance

Is this a brand new instance, or something that used to run fine and has now started failing? If it's been previously running fine have there been any changes made recently?

The OS has failed to boot into multi-user mode, and has dropped into maintenance mode. If this was a physical server or a VM on a hypervisor like VMware you could get in on the console and start troubleshooting, but you're unlikely to be able to do so with an EC2. But look at Method 1 of https://repost.aws/knowledge-center/ec2-linux-emergency-mode just in case your instance is in-scope.

Most likely you'll have to go on and use Method 3 which involves stopping your problem instance and detaching its root volume, then creating a new instance and attaching the problem root volume as the second disk. The you can mount the problem root disk on /mnt/rescue (this is where the knowledge doc suggests) and then start to examine the log files and anything else that could be of interest on the problem server's root disk.

Once you have figured out what's wrong and fixed it, you can detach the problem root from the new EC2, reattach as the first volume of the original EC2, and boot.