Albert Barnes on Acts 20:9

And there sat in a window. The window was left open, probably to avoid the malice of their enemies, who might be disposed otherwise to charge them with holding their assemblies in darkness for purposes of iniquity. The window was a mere opening in the wall to let in light, as there was no glass known at that time; and as the shutters of the window were not closed, there was nothing to prevent Eutychus from falling down.

The third loft. The third story.

And was taken up dead. Some have supposed that he was merely stunned with the fall, and that he was still alive. But the obvious and therefore the safest interpretation is, that he was actually killed by the fall, and was miraculously restored to life. This is an instance of sleeping in public worship that has some apology. The late hour of the night, and the length of the services, were the excuse. But, though the thing is often done now, yet how seldom is a sleeper in a church furnished with an excuse for it. No practice is more shameful, disrespectful, and abominable, than that so common of sleeping in the house of God.