MD5 isn't encryption - it's a one-way hash. You can't reverse a one-way hash (theoretically you can find a plaintext that has an equivalent hash which is generally as good, but you can't in any reasonable amount of time), so you just need to set a new password and email it to them as a temporary, and/or just provide them a link to reset their password.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…