First, turn power off, and don’t turn it back on again until you’ve double checked everything.
The vast majority of issues when you’re getting started happen as the following, in order of how common they are:
* Missing or incomplete solder joints. Check for any solder joints that look dry, cracked, or blobby. Solder joints should form a neat, shiny meniscus without gaps or blobs.
* Short circuits — accidental bridges between neighboring pins or solder dust that makes connections between things.
* Components in the wrong locations
* Missing components
* Components backwards
If it were here on the bench in front of me, those are the things that we would be checking.