The 28-year-old was hit by a straight drive from Sam Hain and was bleeding but managed to walk off the pitch with the help of a physio.
He was taken to hospital and Notts later confirmed he had a concussion.
Players from both sides were visibly shaken by the incident, with some in tears as the match was held up.
The match resumed after a 30-minute delay with Birmingham chasing down Notts' total of 158 to win by six wickets.
Notts head coach Peter Moores came out to talk to his players on the pitch and the players then returned to their respective dressing rooms.
Notts and England bowler Jake Ball posted a picture on Twitter to show his team-mate was in good spirits, apparently in the back of an ambulance.
Moores told BBC Radio Nottinghamshire that it was a tough decision to restart the game.
"The lads were shaken up. I mean one of your mates has just gone down.
"We all sat in there [the dressing room] for 20 minutes. A few lads were struggling a bit, but it seemed the right decision to go back and carry the game on.
"Credit to the umpires, they gave us that half-hour to decide what we were going to do."
Nottingham-born Fletcher made his first-class debut for Notts in 2008 and has taken more than 350 wickets across the various formats of the game.
He was part of the side that earlier this month beat Surrey to win the One-Day Cup at Lord's.
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Nottinghamshire bowler Luke Fletcher was struck in the head by a ball during his side's T20 Blast defeat by Birmingham at Edgbaston.