if we smile during our prayers can we continue or have to start the prayer all over again?
Re: Smiling during namaz

continue your prayer as long as you understand/know what you are doing/saying during salat. laughing out loud nullifies your prayer.
Sadiyah
3
Re: Smiling during namaz
Similing does not invalidate prayer; however, laughing/giggling does.
Source: Ruling on smiling during prayer