Facing the same since yesterday, after a very simple modification, splash screen set to custom mp4 video. Not apparently related to any Wi-Fi settings, actually now reverted but issue persists. In brief, the system (RPi3B with retropie 3.7) is networked (scraper works) but it doesn't reply to any ping or ssh requests from other systems (obviously on the same network). I can ssh the Rpi from the Rpi itself. Repeated the Wi-Fi config following method one in the wiki guide but it didn't solve the remote connectivity issue.
[UPDATE] Resolved after upgrading router firmware. Apparently not a problem of the Rpi nor RetroPie.